@Duncanma: Good to hear you might consider moving it to the Xbox Live ecosystem. And maybe they should change the name from Xbox live to something else, because it's more than just xbox.
Now when (I predict in 3-5 years) the Xbox live team or who controls the achievements contacts you and asks you to transfer those achievements to the Xbox live system, you must be very careful to do it right.
I guess there would be 2 way to do it. Erase the current ones and start over, or do a transfer. I would suggest erasing everyone because of the problems that could come up.
If you do transfer, here are some things to look out for:
You don't want people to be able to copy someone's saved file and have all the cheevos pop on the transfer. I hope you can't do that.
I also hope you are recording the mili-seconds when you get an achievement. Xbox live does and it's one of the ways they catch cheaters. Last year someone with the largest following on TA got caught for cheating which rocked the Xbox live world. It was due to the mili-second timestamp which I heard were all set to 0 on hers. I take no stand on this issue.
If you have achievements in VS2010 and VS2012, do they both count like xbox games in different regions do? I would say yes you need to give double cheevos for those.