Under what cirstances can a prosecutor "amend" a criminal charge?

I went to court today to represent myself in a cl c misdemeanor charge of intoxication. I live in Utah. I did my research well and found numerous examples of both case law which proved that the prosecution had insufficient evidence to meet the necessary standard for a guilty verdict. I spent about a half an hour talk this over with the prosecutor before the trial was to begin. I had him worried and he called his law partner and a drug recognition expert and had a meeting with them. After about 20 minutes the prosecutor informed me that they were going to amend the charge or citation (I forget which they said) from "public intoxication" to "abuse of psychoactive chemical solvent", which is a cl b misdemeanor. I know that the reason he did this was to take away all the ground I had for the original charge and to try to intimidate me into a guilty plea. Anyway, I don't understand if this is allowed seeing as there was no new evidence or anything. The prosecutor said I had no say in the matter (which makes sense) but when we went before the judge, he told him that I had been given the choice and agreed. I objected ad the judge agreed to postpone trial to a later date so I could prepare a defense. I think this new charge isn't even as applicable in this particular case. Thoughts? Suggestions? Was the prosecutor able to do that?