A question on free weapon upgrades

1st playthrough, Act 2, >!Visages!< I've been negligent in upgrading my weapons because I'm periodically finding drops that upgrade weapons i already have and don't want to waste mats if I'm getting a free upgrade anyways (decent amount but not drowning in mats currently, everything I use is 11-14). I know I'm likely needlessly worrying as well probably be drowning the the Resplendent at some point just like I have tons of the basic Chroma Catalyst but nothing to use it on, but just trying to understand the mechanic a bit better. If I have a level 10 weapon and upgrade it to 12, and later find a drop that would have normally taken it from 10->14, does that now instead go 12->14 l, or 12->16? And if the former, if I've already gotten to 14, will it just give me upgrade mats instead, or no benefit?