What happens when a permanent with a modifying layer effect like Kasmina's Transmutation is applied to a creature that already has a power boost?

Magic resolves continuous effects in a strict sequence called the layer system (CR 613.1). Power and toughness modifications are handled in sublayers of layer 7: base P/T is set in layer 7b, power/toughness adjustments (like +X/+X effects) are applied in layer 7c, and counters are counted in layer 7d.

Kasmina's Transmutation sets the enchanted creature's base power and toughness to 1/1 (a layer 7b effect) and removes its other abilities. Because layer 7c is applied after layer 7b, any +X/+X boosts — from Auras, spells, or other static effects granting power boosts — are added on top of the 1/1 base. The creature loses its printed abilities but retains the numerical bonuses from those subsequent layers (CR 613.3, 613.4).

It is important to note that Kasmina's Transmutation also removes the creature's abilities in layer 6 (CR 613.1f). So if the power boost came from the creature's own static ability (e.g., 'this creature gets +2/+2'), that ability is stripped away and the bonus disappears. But if the +X/+X comes from an external source (like Giant Growth or a separate Aura), that effect remains and still applies in layer 7c.

Concrete example: You control a 3/3 Grizzly Bears enchanted with Giant Growth's lingering effect granting +3/+3. Then Kasmina's Transmutation enchants it. In layer 7b, its base becomes 1/1. In layer 7c, the +3/+3 is added. The Bears ends up as a 4/4 with no abilities (other than the reminder text from Transmutation).

References: CR 613.1 (layer system overview), CR 613.3 (order within layers), CR 613.4 (dependency), and the specific sublayer breakdown in CR 613.3c–613.3f for power/toughness.