Service Plan Questions

Background:

Four Phones on the account:

First three bought at the same time and online it shows that the contract on al of them expires on September 20th.

A fourth phone was added four months later and as such it shows that this contract expires in January.

Paying $99 for a basic family plan with 700 minutes, no text messaging.

In September we will be dropping one of the phones.

Now, for my questions:

I would like to get a new phone. Specifically, I would like to get the rumored HTC Hero that is coming out in October. I would also, if possible like to switch us over to a SERO plan, My question is: Do I have to wait until all four phones are out of the service plan before I can do this? And to do that, I would also need to wait to get a new phone once all the phones are out of contract, right?

I'm hoping to find a way around this, but I'm not sure. It seems like the actual service contract, since they're all using the same plan, would expire at the same time.

Thanks for your time.