question re: Shillelagh cast on magic items

[DMSenko]

I know Shillelagh can be cast on a weapon allowing it to be considered magical if it is not already but if the item is already magical does it stack the to hit bonuses, etc.

Druid has a +1 quarter staff & casts Shillelagh - when cast does the +1 stack on top of the bonus of the players spell casting ability score? or is the +1 only valid when using weapon without Shillelagh cast on it?

[JNAProductions]

If it’s already magic, then just increase the damage die to d8 and use casting stat for hit and damage. Other than that, it operates the same as before, complete with magical bonuses.

[Psyren]

The "if it isn't already" makes the intent clear that it will work just fine on clubs/quarterstaffs that are already magical

[KorvinStarmast]

Someties useful to quote the spell text to illustrate a point, such as the one Psyren just made.

Shillelagh | Transmutation cantrip | Casting Time: 1 bonus action
Range: Touch | Components: V, S, M (mistletoe, a shamrock leaf, and a club or quarterstaff)
Duration: 1 minute

The wood of a club or quarterstaff you are holding is imbued with nature’s power. For the duration, you can use your spellcasting ability instead of Strength for the attack and damage rolls of melee attacks using that weapon, and the weapon’s damage die becomes a d8. The weapon also becomes magical, if it isn’t already. The spell ends if you cast it again or if you let go of the weapon.