When is a '72 not a '72?

[primetime]

Just a question, when doing Frankenstein projects and mixing throughout the years, when does the time come that it is no longer the same year c10 you started with? My '72 that I bought from a member here on this forum has gone through some changes. I swaped out the frame from '72 to a '68 already and kept everything else '72. Butttt... I've seen the damage underneath my cab now and thinking I'll be better off with a DIFFERENT cab instead of repairing the '72. If I did this, could I still use my title Oreo I have to burn it and buy a new cab with title? And then the truck becomes whatever year that cab and title is?