I personally keep my silvers until i get a gold that has equally good or better stats on it.
The two advantages Gold moves give you over Silvers is that they start with higher base damage, as well as having higher Maximum Damage at max level.
On top of that they get 3 instead o 2 extra stat increases, which also sclae harder.
Other than the extra Base Damage, you won't get all that much out of Golds that have terrible stats, so i'd say keep the Silvers if you've already leveled them up until yoiu have golds that have good enough stats to switch them out.