I can only think of 2 ways at the moment:
1. Go through your existing layouts with SKILL and round-up/down all geometric information
This is a bit dangerous because it could create opens/shorts due to the rounding process
2. Use VLM (or the optimizer) to perform grid-snapping. (use DRC-fix mode)
This will actually make sure that any snapping does not violate the netlist (and remain DRC correct).
But it could also take some time, depending on the size and complexity of the layouts.
The second alternative may requires some tech-file setup as well. It all depends what info you have in your techlib.
Check the docs for "Virtuoso Layout Migrate" to see if you think this may help.