I registered just to contribute my first hand experience with porting a Verizon number out in order to keep the number while selling the UDP.
First, with regards to the above post abut VZ no longer allowing AOL on UDP, that is only partially true. They closed off AOLs to private individuals, but it is still possible to sell & do an AOL to a business.
Regarding the logistics of porting your number to GV and changing your number on Verizon, I also just completed that successfully using the method described in this thread. The main uncertainty I had was the lack of agreement on EXACTLY WHEN you should request the number change on VZ after initiating the port on GV. I saw some posts that said wait until after 22 hours, and some that said you need only wait until after you receive the GV email confirming that the port had been initiated.
What worked for me was in between: I submitted the number change with VZ about 20 hours after receiving the GV email confirming that the port was in process. I immediately received confirmation from VZ that my number had been changed, and instructions on how to reboot my phone a couple times to activate the new number. That worked fine, although from that point on any calls to my old VZ number received a recording saying the number had been changed or disconnected. During this time my old number was unable to receive calls or texts. I used this time to update GV with my new VZ assigned number, and set call forwarding to that new number so that when the port was completed there was a new call forwarding number.
At 24 hours *to the minute* after the port has been initiated, the GV website (and subsequent email) confirmed that the port was completed successfully! Test calls to my old ported number forwarded via GV to my new VZ number without incident, although texts are still in limbo an hour later, as others indicated to be expected.
I can now complete the AOL on the UDP, and if I wish, add a new line at VZ and port my long time number back. I'll update later to confirm how that process works, and when texts resume.
Thanks to those who have shared this information earlier, or I would have assumed I had to lose my longtime number in order to sell my UDP.