Hey there, recently just bought my first SBR its a complete factory dd, down the road I want to buy a 300 blk SBR upper and maybe some other various SBR-Spr uppers, is it legal to use the dd register lower on other uppers, BTW I am never gonna sell the 10.3, so I can always put it back to what it is register as. Thanks for a newbie.

Yes...

Yes, it's legal, and there's no requirement that you be able to return it to the factory configuration.  Buy as many uppers as you want/can afford.