Change a number, but not lose it

Hello - the subject sums up what I am looking to do.  I would like to change my number, but not lose it. The goal would be to port it to Google Voice.  My understanding is that if I port to GV my line will be cancelled, which is not what I want to do.  Can anyone tell me how this can be accomplished?  Would I actually be better off porting to GV, then get a new BYOD line and add it to my account?  Thanks in advance.