Changing thesis supervisor to avoid bad letter of recommendation from current supervisor?
Delaying your graduation by a semester would be a huge cost to bear due to the mere suspicion of getting a bad recommendation letter.  Right now it's only a suspicion.  

Yes, it's clear that your reputation with your supervisor isn't the greatest.  But still, writing a bad recommendation letter is an exceptional step--especially after being asked and agreeing to write a letter.  Most professors in this situation will write a positive (though perhaps not particularly strong) letter.

Being asked to write a recommendation letter, agreeing without stating any concerns about the content, and then going on to write a bad letter is pretty unprofessional.  If you don't feel you can write a positive letter, the normal response is to not agree to write one, or to agree with a warning that you don't think you can give a good recommendation.  That isn't to say it never happens, but it is pretty unusual.

The examples that show your concern, to me don't indicate that he is planning to write a bad letter. 


The negative general comments are unfortunate but hardly prove he will write a bad letter.
Your idea to report back regularly after you leave his supervision and hope that improves the recommendation was quite odd, and was rightly rejected by him. The recommendation is supposed to be based on the period he worked with you.


Unless there is something more concrete that makes you think you won't get a good letter, I would not do something drastic like delaying your graduation.