In Ontario, I have collector car insurance with Lant, at first it was appraised value, now since merger with Hagerty, it is agreed value. They flipped my appraised value to agreed value, however now only time agreed value changes is if you have something done to the car that increases the value. You can try to get agreed value raised if market value has increased, but sound like the onus is on the owner to do all the homework to justify a higher agreed value. Far as I know the plates have nothing to do with the insurance , I have had regular and now have historic plates, there was no difference or issues . I'm still paying a little over $200 for the year.
Makes me wonder if this agreed value thing is due to appraisers valuing cars much higher than they were worth, I've heard some owners had appraisals where the appraiser did not even look at the car, he/she just asked the owner "how much do you want the appraisal to be?"