Can a postdoc change his/her supervisor in Sweden?
(I'm faculty at Chalmers University of Technology, a well-known private university in Gothenburg, Sweden)
The pragmatic question (whether it's a good idea to change supervisors mid-flight) is covered well by posdef, I'll focus on the legal angle here.
As you mention, PhD students have quite some rights in Sweden, but this is mostly because they are in a special type of employment largely governed by Swedish university law. This is not the case for postdocs - postdocs aren't students and they are not faculty (for which the law also knows some special privileges). Fundamentally, postdocs are just "employees", similar to research engineers, secretaries, or university accountants, and hence their tasks, rights, and privileges are exactly what the university has defined in their job profile. At Chalmers, I am not aware that there is any formalized "right" to be assigned a different supervisor. I suspect the situation in other universities is similar, but you may want to check your own contract first.
That said, and to echo the other answers a little, changing your supervisor is hard. Really hard. Even as a PhD student, when the law is on your side. Funding is a huge issue, but so is the fact that faculty tends to be sceptical of taking on students/employees where one of their colleagues has already made bad experiences. As always, there are exceptions, but in general you should figure out which faculty would take you on and take over your funding before talking publicly about changing supervisors. It's better to have a normal working relation with a supervisor that is not ideal, than to have an advisor who has been assigned by the department because nobody else took you on.
TLDR: It will depend very heavily on the funding situation.
A reasonable guess as to why there is a clear remark about the PhD students and not about the postdocs is that how postdocs are employed varies significantly.
Without having any proof, I would guess that you can't change your mentor however you like if s/he is paying you. Because why would you have that right, it is that person's own funding. In that situation your best bet (besides trying to find a resolution) is to quit and apply for a new position. Or better yet, apply for new positions and quit when you find one.
If you are bringing your own money, then you would have more flexibility, although it would still be an unpleasant move if you just bail out and start working down the aisle or the next floor.
That being said, you can in general not be forced to submit an article you are not happy with. Because you can always get it pulled (or your name removed) if you are not OK with it's content. That creates bad rep for the group and the paper in question, so I would be willing to guess that it's not really a common situation. More likely, you and the PI has differences of opinion as to why the paper should (or should not) be submitted, and if that's the case it's generally a better idea to try to understand each other rather than fight it.
You can however be forced not to submit a manuscript, or prevented from going to a conference, and I would say it happens relatively often. Usually not it harsh terms, but also that sometimes. That is part of the employment, that would happen anywhere and it's important to try and find constructive solutions. The comment by @lighthouse_keeper is on point, if you try to go down that route, you are burning bridges and you are the small fish in this particular case.
EDIT following OPs comment:
Re: the role of a postdoc
When a group leader gets funding, it's typically from a grant (exceptions exist but lets put that aside for the moment). Grants are written for specific subjects with usually well-defined tasks and goals. In the context of these tasks and goals, one or more postdoc positions might be opened.
So when you are a postdoc, you are taking on a role in that lab that fits their goals and targets, with the implicit assumption that whats good for the lab is also good for you. In other words, it's implied that carrying out your research project there is beneficial for both parties, meaning you choose to do your post-doctoral research there. If that is not the case then obviously it's not a good situation.
Within the context of that project and academic code of conduct, the group leader does have the privilege to direct the project as s/he sees fit. If your disagreements are fundamentally scientific (i.e. not related to bullying, fraud or something similar, which would typically violate the university's code of conduct) I don't see how you'd argue for a valid reason to change to another group, while keeping your current position.
Re: the comparison to a PhD student position
In comparison to a postdoc position, the university and the faculty is an active stakeholder when a PhD position is announced. In Sweden, most (all except a couple) universities are government institutions. So by being a fully employed PhD student you are a state employee, besides being a student. In this case the university (and by extension the state) has accepted you to a degree of higher education. If your supervisor cannot teach you well, for one reason or another, you are entitled (within reason) to ask for a change.
As you see there are some fundamental differences in what the employment is, what is being offered. I would not necessarily say that your rights are more limited, essentially it's not really a "right" to be able to change your manager. Your rights as it pertains to social security, healthcare, insurance, paid holidays, paid parental/sick leave etc apply regardless if you are PhD student or postdoc, as long as you are actually employed and not on a stipend.
Just to add a few things to posdef's great answer. I think it's worth mentioning that a PhD and a postdoc position are very different in terms of work dynamics with the "supervisor", and trying to compare the two is a bit questionable:
- The primary purpose of a PhD student position is to provide the student with specialized research training under the supervision of the PhD advisor. Of course the PhD student carries out research work, but they are not considered autonomous in this work that's why they are supposed to work under the direction of the PhD supervisor. As a student whose academic success strongly depends on their supervision, they are entitled to specific protections.
- For all means and purposes, a postdoc position is a work contract: the employee carries out a job for a Principal Investigator (PI). The role of the PI in this work relationship shares some similarities with the role of a PhD supervisor but it is significantly different:
- The postdoc is supposed to be autonomous in their work and as such is normally entitled to greater academic freedom than a PhD student. Ideally, in a healthy postdoc-PI relationship the two work practically like collaborators (as opposed to student/professor). For example the discussion about whether or not to submit a paper that both co-author should be made by mutual agreement, because the postdoc is able to make their own academic choices.
- Ultimately the PI is usually the one who controls the funding, and the institution cannot force the PI to give away their funding. As a consequence there's no general mechanism to "change PI", since the institution doesn't have any alternative source of funding to pay the postdoc. This is a major difference with a PhD position, where the funding is usually allocated to the PhD student themselves.