Roaming is available on the prepaid One plan and it works in Mexico and Canada but you might want to look at the other international roaming rates before you jump on this plan. http://tmo-assets-1524598101.netdna-ssl.com/prepaid-phones/pdf/IntlCallingRates.pdf
One option is to get your phone unlocked and use a prepaid roaming service like Piranha Mobile. Depending on your roaming needs this may be more cost-effective than changing your T-Mobile plan. Be careful to verify that your phone is actually compatible with the cellular systems in the countries you plan to visit. In the Caribbean, this should not be an issue but some Asian countries (e.g. Singapore, Japan, S. Korea & China) may not play well with some otherwise "international" phones.