When did Fender originally switch from nitro to poly? Did they change at the same time for all guitars or did they roll them out at different times?

Everything I've read over the years says the poly was transitioned in during 1968. I would assume all models similarly - but you could wind up with one with all nitro, one with a nitro neck/poly body, or one with all poly at different points in the year depending on how the individual model (Strat, Tele, Jazzmaster, Jaguar, Mustang, etc.) were selling at that time and when they ran out of stockpiled nitro-finished parts. Surely by sometime in early 1969 all models were all poly.

But being Fender I'm sure someone out there has an exception to just about any "rule" for spec transitions.

Also a fun fact - for several years the fronts of the headstocks were still finished in nitro because the poly they were using reacted with the headstock decals (and the decals were now under the finish as of whenever the large CBS black decals started being used in 1968).

Just an addition to John C's fine post.

Fender started using a form of poly as a base coat around the end of '63. Even today, the nitro-finished guitars have this poly underneath, and AFAIK there are no all nitro finishes anymore, except maybe special orders (Wildwood?).

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This is true, even the "thin skin" nitro guitars today have a poly sealer. However, Fender never made a guitar that was 100% nitro. Even those made in the 50's and 60's. Confirmed this w/ Fender.