Is it unethical to edit the text of my undergraduate thesis prior to sharing online?

IMHO it depends on why you want to make it available.

If your primary reason is that you want to make information available more publicly, you could produce a "revised edition" of the thesis and add a preface stating that this is a substantially revised and edited version of your thesis - just like other books do. If you just correct typos and grammar, just say that.
There is also the possibility to comment on the conclusions you don't see like that any more in a way (formatting, footnote) that makes clear that this is a point where your opinion now differs. (like a commented edition of some work).

If your primary reason is to give the text your online CV refers to, then leave it unchanged.

Is it unethical to make considerable changes to the text

No, it's not unethical, but if you make changes it would be useful to include a note or a preface disclosing that that is an amended version of your original thesis, and if your conclusions have changed considerably after further experience, you can explain why.

The important thing in such matters is to be honest.

Releasing a revised version clearly marked as such is perfectly ethical.

Releasing a revised version while leading people to belive it's the original thesis that you submitted to the university years ago would IMO be unethical.