The activated ability will overwrite any previous effects that set the creature's power and toughness to specific values. Effects that otherwise modify the target creature's power and toughness will still apply no matter when they took effect. The same is true for +1/+1 counters.
2021-07-23
Minsc has asked us (repeatedly) to let you know that Boo is the only miniature giant space hamster in the Realms.
When Minsc enters, create Boo, a legendary 1/1 red Hamster creature token with trample and haste.
: Until end of turn, target creature you control has base power and toughness X/X and becomes a Giant in addition to its other types. Activate only as a sorcery.
: Until end of turn, target creature you control has base power and toughness X/X and becomes a Giant in addition to its other types. Activate only as a sorcery.