I’ve rigged my character and then realized I made some mistake which moved some vertices on the left side. Normally I would just delete the left half and apply a mirror modifier, but now I don’t want to rerig the left half! So what can I do? Can I copy the weights from the right side bones to the left side somehow?
The mirror modifier also mirrors the vertex groups (if you have the right naming convention, i.e. the groups ends with .R/.L, or any of the supported alternatives). Just make sure that the Vertex Group option is checked in the modifier, and that all vertex groups exists (both the .R and .L groups must be there, since the modifier won’t create any new groups).
So you can actually keep the mirror modifier even if it’s rigged.