Question on Unlocking HTC Thunderbolt ( no longer in contract )


I have a HTC Thunderbolt and my contract with Verizon has expired about a year ago (I now pay month to month).  Now that I have satisfied the contract the phone is completely mine.

I am curious as to how to go about unlocking the phone ... And if I did so, what happens immediately (meaning will I automatically no longer have Verizon service?) What if i choose to keep service with Verizon?  It would make sense to me that I should be able to used an unlocked phone on Verizon, especially when the phone was on the Verizon network for three plus years now.  Please help me to understand. I have scoured the support site with no luck, which is a bit surprising to me (that other folks are not interested in unlocking a phone that they have essentially paid for)

I appreciate any concrete assistance I can get.

Before anyone questions my judgement, I do like my phone. With one exception of a drained battery issue that surfaced coincidentally at the same time as the ICS update, this phone has not let me down.

Thanks in advance!