Can a bank change a legal document without my permission?

I agree.  The issue that may arise is if you saw the change and did nothing about it.

Did you plead fraud?

Can you prove that the original document clearly stated that the LLC, not yourself personally, was the guarantor (e.g. do you have your own copy of the original)? If you can prove that the bank changed the document AFTER you signed it, then that should be fraud--not only can they not enforce the debt against you personally, but you could either sue them for fraud and/or report them to law enforcement (or bank regulators) as fraud.

However, if either (1) you were suppposed to guarantee personally and they can prove that you were (and knew it), then they could show that writing in the LLC as a guarantor was your improper attempt to avoid liability you'd agreed to; or (2) if the change was made before you signed and you signed anyway, then clearly you did assume the obligation.

So the question is what the factual record shows about which person or entity was supposed to make the guarantee and the timing of changes. If the record is in your favor, you could, as noted above, report to regulators and law enforcement.