Alright, the lower is what's actually SBR'd. The upper is not serialized; therefore it can't be registered. All relevant information must be engraved on the lower (there are also rules on how deep the engraving must be and the size of the text). It'll look a little something like this:
Joe Blow
Some town, Florida
Once you get your tax stamp back, you can switch uppers to your heart's content. However, it's wise to keep the original upper that was intended for it per the forms whenever you were filling it out. If you happen to make a permanent change to your SBR, you must report it to the NFA branch so that they could update the database. For example, you described your SBR as a 10.5" with an overall length of 29'. If you decide to put a 14.5" on there instead, just tell the ATF.
If I were you, I'd let a friend hold the 14.5" upper for the time being as the ATF would jack you up for "constructive intent" if they were to go to your residence for some strange reason. It's just a good way of covering your ass until you decide what you would like to do.
ETA: 10.5" greatness.