Luckily not often, but I do remember a few occasions that I answered a question, and the poster decided afterwards he or she actually meant to ask another question.
While the posted answer actually was an appropriate answer to the original question, it looks foolish on the editied version, and you may doubt if you would have answered the question in the first place, or the answer would at least be quite different.
Th direct reason is this question, originally named:
Which file(s) generate window titles?
Which files are involved in generating the window title? Fx. if you open Rhythmbox, you see that as a title. But it's not generated by the .desktop file. Does anyone know which file(s) I need to edit so I can add text to the title of an application?
This is quite different from:
How do I permanently change window titles?
In the original question was no sign whatsoever OP meant to interfere in a permanent way on the window name on a specific application. Adding text seems an occasional thing to do, and I answered accordingly.
After some edits however, the poster changed the meaning of the question, making the original answer look misplaced.
Is it acceptable to drastically change the title / meaning of a question after answer(s) have been posted?
I would say we shouldn't. It makes you feel awkward. Not only early posted answers will possibly (and likely) look foolish, but the findability of those answers will be decreased (in general, not specifically this occasion). A better option for OP would IMO be to accept the answer that works for him or her and motivate why it works in the specific situation.
What is the generasl idea/opinion?