First, for folks who haven't seen these watches, here are pics:


I'm really glad you asked this question. I've wondered the very same thing. A related question I had involved the serial number. I wondered how Omega would deal with the serial number issue when they replaced the movement. On 2/23/08, I asked this question of someone who actually had this done. He said:
When Omega replaced the movement, they did assign a new SN. I was surprised to see that they removed the old number and engraved a new one on the case as well.So your original papers, if you still have them, would not match the watch after getting the movement replaced. Personally, that would bug me unless I had a 2500A movement that was a "lemon" (in which case functional concerns would be paramount).
I'm sure people will have different opinions about this subject. Personally, if the original movement functions okay, I'd leave it alone. I'm not saying this because I'm a purist when it comes to these watches. In fact, the white metal watch shown above is actually PT, even though it's shown with the silver dial that came with the WG (not PT) version. The original PT model had a brownish dial that I thought was really ugly. I got a replacement dial in silver and had it changed ONLY after being reassured by my watchmaker that I'd be able to keep the original brownish dial as a spare part, so the watch could be restored to its original state later if desired. But when you send it to Omega to have the movement changed out, keeping the original movement probably wouldn't be an option, since (a) Omega might not agree to return your original 2500A movement to you (do folks think Omega might agree to this??), and (b) Omega will change the SN on the case (unless you can convince them not to??).