Actually KTM will be racing the SX250F or whatever they call it and all they need to do for the production rule is sell 400 of them by June 1st (I think that's the date), even though they will start racing it 6 months earlier.
Yamaha etc. can do the same, but I think the riders don't like to make changes mid season. Ty Davis is running his 04 forks on his 05 framed YZ, cause he did not have enough time to test the 05 fork.
MC raced a steel framed CR after the 97's were already on dealer floors cause he thought the new frame tired him out too much.
RC raced his previous year CR250 when the new powervavle equiped CR came out, cause he didn't like it.
So basically the Pro's will race the new stuff early if they like it, and if they don't they will not. They want time to test it and they must feel better and be faster or they will not switch.
You forgot to mention that MC rode with the 92 frame on his 93,94,95,96 bikes