The "old" SIMs do work, at least when used in unlocked phones direct from Apple. When my husband's unlocked T Mobile phone arrived a week or so ago, all we did was switch the SIM in from his still locked under contract AT&T iPhone 5. ZERO issues, the new phone worked right away and he's had no problems with service.
So I'm a bit surprised that the OP's phone didn't simply activate.
he likely bought it from the AT&T store with a new 2 year agreement or payments
you want to hear something really crazy? if you have not heard already. if you stick a pre-activated verizon sim in an unlocked iPhone 6, it works, and it becomes a verizon phone and CDMA calling works. and all the other features work as well like VoLTE and Vsiaul voice mail.