I used to have Verizon. Before I left them to go to BoostMobile, I ported my cell number to Google Voice. Then, when I bought my BoostMobile phone, I went to Google Voice and added the new cell phone's number (one that came with the BoostMobile phone) to the list of phones that Google Voice will forward calls to (old number forwards to as many phones as you like!). To make outgoing calls use your Google Voice number (and not your actual cell number - BoostMobile's in this case), you install the Google Voice app on your phone. It routes your outbound call to reflect your Google Voice number.
Now cool thing is that I had to change carriers again... now I use Straight Talk's Proclaim (Verizon towers vs Sprint towers on BoostMobile) and was able to have Google Voice ring BOTH phones (for the remainder of the BoostMobile service)... so I never missed a call.
I highly recommend you just jump in and try it. The most awesome part of it all, is that you can buy an ObiHai box (appox $50 at Amazon) and get your Google Voice number to also ring normal home phones in your house... Never pay for a home phone line again! Someone calls your old cell number (which is now on Google Voice) and your cell AND home phone ring... answer whichever you like... they can always get you. == The home phone option was great for me since the cell coverage in the house sucks.
Still nervous? No problem. Just go ahead and set up Google Voice. Use a number they provide... you don't need to port your number right away. Have it forward to your cell and play with the Google Voice app... and home phone... and when you are done being amazed, port your cell number to Google Voice. If for some strange reason you don't like it... then no harm, no foul. Just stop using it before you port your number.