If the charged with Possession of a dangerous is the charge, but it was an illegal weapon you had. Can they change the charge from what they submitted

I am being charged with Possession of a Dangerous weapon under C.R.S. 18-12-102, However I did not have anything on my person that is definable under the statute. I did have a pair of brass knuckles as defined under "illegal weapon", but the officer DID NOT cite it under illegal. The ONLY defendant is myself. I was stopped for lack of license plates, officer asked for License, Reg, Insurance. I demanded to know what crime he was suspecting me of? and to obtain a warrant before searching my property for private paperwork. (please leave the laughter out of the response LOL), but he then opened the door, arrested me for obstruction, citing 18-8-104, but I am understanding that requires more then a verbal obstacle. He then found my brass knuckles, which were used as a necklace, but I was just coming out of work. Now, since the original infraction was not a crime, under 16-3-103(1) they had no valid reason for stopping me in the first place.