Not sure if it's the same for US ones, but the network may let you convert the contract SIM to a pay-as-you-go one, keeping the same number. They'll still smack you for roaming etc though.
Another option, grab yourself a Skype number and start giving that number to your contacts. Then when you're abroad it won't matter what cell number the network allocates to the SIMs, as long as you've got some data allowance/coverage your contacts will be able to reach you using the same number all the time.