What is a development branch?
The development release is not treated as 'stable'; it won't be treated as stable until release date.
Ubuntu 18.04 LTS users won't be offered to upgrade at that time though, as LTS users are seen as wanting the most stability. Instead it'll only be offered for upgrade to 18.04 LTS releases when Ubuntu 20.04 reaches the 20.04.1 milestone.
https://wiki.ubuntu.com/BionicBeaver/ReleaseSchedule
Current date of Ubuntu 20.04 LTS release - 26 April 2020
Current date of Ubuntu 20.04.1 LTS release - 26 July 2020 (ie. this--or normally a couple of days after--is when existing Ubuntu 18.04 LTS users will be offered the upgrade; with the delay to ensure stability for LTS only users)
Development releases have more updates, more reboots required, and yes, because it's a development release and thus still in testing, it may not be stable. Not all users may experience instability (it can depend on software they use, hardware or a number of factors), but myself using the development release for testing purposes since 17.10 have had only 3-4 issues.
I dual boot, and my other OS choice is Ubuntu 18.04 LTS, so if I have issues with this 20.04 development release, I can fall back to 18.04 on the same disk to correct any issues (or use a 'live' thumb-drive) though I can't remember needing to do that in more than a year (or when 19.04 was development, 19.10 and now 20.04). About a week after 20.04 is released, I'll bump to 20.10 staying on development as it's useful for me as a development team member (a tester).
Other parts to your question
Your development release will automatically turn into the released version as long as you keep upgrading, it'll turn to the full release in the hours before ISO becomes available most probably.
Can you upgrade to the latest non-LTS release easily? No, the next non-LTS release is actually 20.10 as far as 20.04 is concerned, which won't be started until 20.04's actual release. Ubuntu tools for upgrade work to 'upgrade', not 'downgrade' so whilst it's technically possible, it'd take a lot more work (and many times longer) than a re-install would.
Please note: Ubuntu 20.04 support questions cannot be asked on this site though as it's currently a development release. Ubuntu Forums, and IRC have ubuntu+1 areas for development release support requests; so your support options are reduced. Being a development release, many problems encountered belong instead in bug reports so any questions closed will suggest filing bugs so they can be fixed.