Replace selected Objects with new objects?????

[Brian Jones]

use the built-in Transfer function, there are plugins too, Other Objects PSR comes to mind…

[Greg Serafin]

If you are talking about using stand-ins (as in, using a simplified model to set-up the animation and then replace it with the complex one for final rendering) try a plugin called something like “clone animation”.
I remember having some luck with it a while ago. It will not clone deformers (but they can be copied from one object to another) or PLA animation, but the rest worked OK.
The other way I deal with using stand-ins is to child a complex model to a skeletal version of it, make it invisible, animate the simple one (just to speed up the screen display) and for the final render make the simple one invisible and render the complex one.

IQ