Since you have an iPad you can continue using iMessage, but only with other iOS devices as pointed out above. Your iPad can use iMessage independently of your phone number, which sounds like you're in the process of removing from association with your Apple ID. Removing your phone number can take awhile, so while that may play into your favor for continuing to use iMessage for your iPad, the number could get disassociated while you're overseas and iMessage (from your friends' perspective) may fail.
So to plan ahead, since your iPad can send and receive iMessages from an e-mail address, what I'd recommend is, on your iPad go into Settings/Messages/Send & Receive, then choose your email address in both the "You can be reached at .." and "Start new conversations from .." sections.
Then, for the people you want to communicate with while overseas, start a new message thread from your iPad (don't reply to an existing thread) with each person, instructing them to reply to this new message thread while you're away, not older messages which may be associated with your phone number. While you're overseas you'll get your iMessages just fine as long as you have Wi-Fi access.
Also, do NOT turn iMessage to OFF on your iPad since messages that come in while iMessage is off will fail and not be re-sent. But if you're out of Wi-Fi range, you'll get the iMessage when you get back in range as long as you leave iMessage ON.
