Hypothetically, it might be possible, but there aren't practical gains to be had from it. Easier to fabricate a new chip.

The optimal design is unlikely to be a straightforward modification of an older chip.

Even if it were, modification would add more variables, where we're constrained by yields today.

And our processes today aren't designed for it.

EDIT: That being said, you might be able to repurpose an older chip in a piece of electronics that uses multiple chips in a situation where you're unable to get the older chip for some reason. I can't think of many scenarios where that'd be economical, but hobbyists have certainly reused components from older electronic devices.