As I am about to upgrade to the Galaxy S3, my brother will be taking my Captivate. I've installed Mosaic IX on the phone and he wants to keep his number, so how can I reset the phone so the AT&T employees will not know it's been anything but stock?
1. Are you both currently on AT&T? If so, he should be able to put his SIM card into your Captivate and use it right away. The only thing to look out for is that they'll add a data plan when he puts it in.
2. Where would the AT&T employee see that the phone was something other than stock? The only time I've ever seen this really mattering is when you're getting warranty service. If that comes up, just flash back to a full stock ROM and you should be fine.
1. Yes we are, but he has an iPhone 3GS, so I don't know if that affects anything. But the iPhone data plan is a separate data plan than regular data if I'm not mistaken...
2. If we all bring our phones into the store, because of me buying the Galaxy S3 and getting a new micro SIM, I'm worried maybe when we swap phones and get it arranged, perhaps they may discover it...if they do though, what can happen, if anything?
2. Probably nothing, but if you're really worried about it and you've already gotten the data off of it, just go here and get either the I897UCJF6 or I897UCKB2 one-click and flash that. It'll put the phone back to complete stock (but won't wipe your personal data like pictures, music, etc off of the internal card).
If yours is saying I9000 when you first boot, make sure you get a package with I897 bootloaders (if the stock issue worries you). Both the ones I linked to are capable of flashing those bootloaders.
To answer your question, it will flash backwards fine, but they could potentially tell that you have the I9000 bootloaders on there.