It depends on what you want to do. If you want your current number as your GV number you need to port, but you'll probably lose unlimited data and may end up with an ETF (the link above showed how you can avoid the ETF, but it seems like a lot of work to make sure it happens correctly).
What I did was use the GV number search tool to get a number very close to my mobile number. I started giving out my GV number and updated my friends' contact information. To activate it as your voicemail just dial *71 followed by your GV number and hit send (*717325551234 if your GV number is 732-555-1234). This way your phone contract info doesn't change but you do need to get people to start calling your GV number instead of your mobile if you care about other GV features like ringing multiple lines or SMS. Some more info on the call forwarding:
Calling Features: No Answer/Busy Transfer
What I did was use the GV number search tool to get a number very close to my mobile number. I started giving out my GV number and updated my friends' contact information. To activate it as your voicemail just dial *71 followed by your GV number and hit send (*717325551234 if your GV number is 732-555-1234). This way your phone contract info doesn't change but you do need to get people to start calling your GV number instead of your mobile if you care about other GV features like ringing multiple lines or SMS. Some more info on the call forwarding:
Calling Features: No Answer/Busy Transfer