Allow guests to upgrade or downgrade purchased upsells

Currently, if a guest purchases an upsell (e.g., 1-hour early check-in) and later wants a different option (e.g., 2-hour early check-in), there is no way to upgrade or downgrade the selection with a price adjustment.

Reasoning:

• Reduced Friction: This would eliminate the need for repeated back-and-forth communication between the guest and host to cancel/reimburse one option before purchasing another.
• Host Control: Hosts need the ability to enable or disable upgrade/downgrade options. This is critical because some higher-tier upsells may block off dates/time on the calendar; without the ability to limit downgrades, a host could lose revenue on a blocked date if a guest downgrades to a lower-tier option.