Yes you can add/remove features at ANY time with no penalties. You can also change your rate plan or get a new phone at ANY time. The only difference is if your under contract and want a new phone it wont be as discounted as it would be not under contract. You have 14 days buyer remorse from the retail store and 20 days from tmobile.com from the day you recieve it.. Make good choices!
And FYI, the sidekick plan works with any
gsm phone and is unlimited everything excluding minutes..