Can I use water-based paint over solvent-based paint? Is it OK to use solvent-based paint over water-based paint?

There is no particular problem because an undercoat (sealer) is applied when water-based paint is used over solvent-based paint.
When solvent-based paint is used over water-based paint, the solvent may damage the old paint film and cause abnormalities such as blisters and shrinkage. Check the condition by test application prior to the actual application.