Just wanted to pop back here to say thank you for the help and document my experience in case it helps others.
I was able to get my Captain's Club number setup with Select status by calling Celebrity directly. They used my Royal Caribbean Crown & Anchor number to verify my C&A status - then matched the level for CC (they can also look up your C&A info using name, date of birth, etc., if you don't know your C&A number). Celebrity put a hold on a V2 for me at V3 price. Knowing what my rate should be, I then spoke with a travel agent who called Celebrity to get the same upgrade and book me a cabin - again got a V2 for the V3 price but since I went through the agent, I also got their onboard credit promotion. They couldn't "takeover" the booking that was on hold directly with Celebrity (or maybe didn't try since I've had agents takeover bookings I've had with Royal Caribbean in the past) - so we got a different cabin number - but we're happy with the results overall.
Thanks again for the help.