If I have mistakenly drawn something in rmetal1 and I want to change all rmetal1 in the box to metal1, is there a command to do that?

Not in general, but in this specific case,

cif paint MET1 m1

will do it, because drawing metal1 over rmetal1 will erase the rmetal1, but in GDS, rmetal1 is just the MET1 mask layer.