No rules. Do whatever pleases you. If you have something genuine original, I would retain the parts to keep it complete.

Examples of what I consider upgrades

I'm two projects away of building up an 80's steel lugged Italian frame with what most would consider mid level Suntour goodies. Its going to be bullet'eproof. I also like Campagnolo Velox, various steel hubs, obscure derailleurs, etc..

Yet I like seeing others using modern high zoot carbon bits, wicked lightweight cranks, ultralight levers, trick fasteners, whatever.