The spell simply says “you touch a non-magical weapon” and it becomes magical for up to an hour (concentration required) with a +1 bonus. If you cast it with a 4th level slot, you get a +2, and with a 6th level slot, a +3.
Since there are +1/+2/+3 buffs also associated with this spell, I was wondering if you could buff an already-magical weapon temporarily with this spell. Not stacking, but just changing a +1 weapon to a +2 for a minute, for example.
I suspect this almost never comes up, but I just was curious about the somewhat-ambiguous wording since there’s no explicit restrictive wording that it ONLY works on non-magical weapons.