can I use old field elements in my robot?
For almost all elements no.
But for some things like the platforms from last year that use legal vex plastic then yes.
im trying to use the base of the mobile goals from last year the black part as a wheel of sorts
If you are competing in VRC then no.
Lots a IQ parts get pushed in a closet or in a room. I would try to make projects with the past game elements and present them in a show or something.