Conceptual Problem: Subscriptions

Hello,

I'm trying to figure out how to handle site subscriptions.

I want the user to be able to:
1. Create a new subscription.
2. Upgrade options on a current / future subscription.
3. Extend the date of their current subscription.

I'm wracking my brain trying to figure out how to implement this.

I'm obviously going to have a subscriptions table, but should each subscription actually consist of multiple entries with different start / stop dates?

I'm finding problems each time I come up with a solution. Let's say the user has a basic subscription and then wants to upgrade or extend it. How do I keep track of the fact that their subscription now needs additional payment, but still allow it to continue as-is until they do so? I'd have multiple overlapping (start/stop dates the same) subscription entries.

Has anyone implemented this? Is their an easier way?

Are you talking about MS-SQL replication?