We're glad you're thinking of joining TuneCore!
In order to switch to TuneCore from another distributor, you will need to contact your current distributor and request that they take your content down from stores, and then you can re-upload and distribute it again through your TuneCore account. Unfortunately there is no way to move content on the back-end so it remains live in stores while you switch distributors.
When switching over from another distributor, there is always going to be either a "gap" or an "overlap" of some kind between the time your old distributed content is removed and your new content is live. This is because each store has a review process.
You can use the same UPC code for your album, and same Song ISRCs for your individual tracks as long as the album you're redistributing is exactly the same. If the album is changing in any way (you've added or removed some tracks, or tracks are in a different order) you should not use the same UPC. If the individual tracks have changed (you've re-recorded the song, remixed the audio, changed the title) you should not use the same ISRC. If you a need new UPC or ISRC, just leave the optional fields blank and we will generate new ones for you for free .
If you're okay with having a little bit of an overlap, you can go ahead and start setting up your TuneCore account whenever you're ready, begin uploading your artwork, audio, etc., and selecting the stores you want to deliver to.
Request a take down by your old distributor and distribute your album through us at the same time. You may have a few days where both versions of the album appear in a few of the stores, and a shorter gap between when your old version comes down and your new version goes live in iTunes.