I don't get it: what is the problem?
When user is ready to travel overseas, he should connect his T-Mobile blackberry to desktop through USB cable, and start BlackBerry Desktop software. It should recognize new device and offer to use new PIN for user's profile.
After few minutes of syncronization T-Mobile blackberry will become primary device for that user, with all emails coming to and from it using primary user's email address, with calendar syncronized.
When user comes back, he should repeat this with his VZW blackberry and he will switch to that device as his primary.