Migrate subscription

Migrates a running subscription to a later version of its current plan. If startingPhase is omitted and billingAnchor is omitted or unchanged, the subscription is amended in place. Unchanged items retain their service periods. Changed or removed items end at the effective time; replacements and additions start from that time. For in-place migrations, existing addons must be compatible with the target plan version. The phase timeline, billing anchor, and subscription-level billing settings are preserved. Both response entries refer to the same subscription: current is the before snapshot and next is the amended view. Providing startingPhase or a different billingAnchor uses subscription change: the current subscription ends and a replacement starts at the effective time. This resets the phase timeline and can produce billing adjustments. Addons are not transferred to the replacement.