Samsung BOGO - Swapping SIM Cards to Keep Phone Number

I've combed the internet and this site for an answer but not finding anything definitive.

We did the Samsung BOGO (that required opening a new line) and I'm trying to keep my old number on the new phone.  Maintaining the new line/number is no problem, I would just like to use that number on another (older) phone.  As I understand, putting my Note 4 SIM card (with my number) in the new one ought to take care of that, but our AT&T rep in the store (and various results around the web) seemed uncertain as to whether this would mess with the monthly credit that is applied with the BOGO deal.  Some things have suggested after the 3 months and the credit has kicked in, it should be safe, others have expressed uncertainty.

Anyone know for sure?

If a SIM card swap won't mess with the credit (again, we will maintain service on the new number/line), is it safe to do before the credit starts applying to the bill?  I'd really like to be using the new phone full time ASAP, but I'll wait if necessary.