What happens when a player casts a spell in a layer that modifies another continuous effect already in a higher layer?

Continuous effects are applied in a strict sequence of seven layers (layer 1 through layer 7), as defined in CR 613.1. Each layer handles a specific kind of modification: layer 4 covers type-changing effects, layer 6 covers ability-adding/removing effects, layer 7 covers power/toughness, and so on. Effects are applied in layer order every time the game state is evaluated.

When a new spell or ability creates a continuous effect, that effect is slotted into whichever layer is appropriate for what it does. It does not 'reach back' and alter effects already processed in higher-numbered layers. Instead, the game simply re-evaluates all applicable effects in layer order from layer 1 onward. The result can change because a new effect now participates in the calculation, but no effect is retroactively modified — they are all just re-applied in sequence (CR 613.2, 613.6).

Within the same layer, effects are generally applied in timestamp order (CR 613.7a), with dependency exceptions handled first (CR 613.8). A spell cast later typically has a newer timestamp and therefore applies after earlier effects in the same layer, potentially overwriting them if both affect the same characteristic.

Concrete example: Your opponent controls a Grizzly Bears (2/2) enchanted with Giant Growth's one-turn +3/+3 effect (layer 7d, timestamp 1). You then cast an effect that makes all creatures base power and toughness 1/1 (layer 7b, timestamp 2). Re-evaluating: layer 7b sets the bear to 1/1, then layer 7d adds +3/+3, making it 4/4. The new effect entered layer 7b but did not erase the +3/+3 effect; both are simply re-applied in sublayer order.

Key rules to remember: CR 613.1 defines the seven layers, CR 613.2 states effects apply in layer order, CR 613.7 governs timestamp order within a layer, and CR 613.8 covers dependency (where one effect's existence or extent is determined by another), which can override timestamp order in exceptional cases.