It's just a coincidence that 906 and 907 are one digit apart. Google Voice only supports numbers located within the 48 contiguous US states (so, not in Alaska, Hawaii, Puerto Rico, Guam, USVI, etc.).
As I mentioned previously number portability into GV is based not on area code, but at a much lower level, down to the nine-thousand+ number block level.
There is no way to guarantee or even guess whether your number will be portable until you roll the dice and port it over to GoPhone, and wait a week. Sorry, but number portability is a complex subject, and it depends on Google's telephone carrier partners, who are independent companies with their own equipment and presence (or no presence) in various exchanges around the country.
If you do decide to go for it, and you do use GoPhone, one crucial step is that you will need to contact GoPhone customer care when you are ready to port, and specifically ask them for the account number to use for porting purposes. It is not displayed anywhere visible to you -- you need to ask for it. Also, the PIN you will need is the PIN used to access your account details on their website.