Seeking Lenovo / threadripper advice

I'm loath to give Apple more money, and considering upgrading to a Lenovo p620 with threadripper. I'll repurpose my nvidia 1080ti 's for now, to keep the costs down. My big question for the group, and I can't find this answer with google (my google jedi is weak, apparently): Is the socket the same for the 3945wx as it is for the 3975wx and/or 3995? In other words, if I start with a 55 to save money (16 core,) can I swap in a 75 or 95 later? (32 or 64 core.)
Any other tips on switching from mac to windows much appreciated.! For the price of the threadripper lenovo, I can get a used 2017 imac pro with an inferior GPU. My main apps are Resolve, then Avid media composer, then I'm forced by clients to use premiere from time to time.
tia
Any other tips on switching from mac to windows much appreciated.! For the price of the threadripper lenovo, I can get a used 2017 imac pro with an inferior GPU. My main apps are Resolve, then Avid media composer, then I'm forced by clients to use premiere from time to time.
tia