Howdy: your assumption is correct: When you authorize an OBiTALK device to use the Google Voice service on your Google/Gmail account, you're doing exactly that: authorizing the account, not the phone number. So, no changes should be necessary on the OBiTALK side.
On the Google Voice side, if your original Google Voice number was obtained from Google's pool of freebie numbers, then, after you port in your number, that original number will go on "death row" for 90 days. This is designed to give users time to advise all of their contacts of the new number. After 90 days, that number will disappear, unless you want to keep it. If so, you'll need to pay a second fee.