As long as the SIM free phone isn't locked to a particular network, then yes you should be able to put in his current SIM and the phone should work.
If it is locked to a particular network, them you can get it unlocked. This can be by getting a code from various online websites or having to take it somewhere to do this, but if it's just a case of waiting a few days to transfer the number across, I guess you won't bother.
Btw, I transferred my number from virgin to giffgaff a while ago and it all went smoothly. I asked for the PAC from virgin on a Thursday, and they gave it to me straight away. Told Giffgaff that afternoon and they changed me over sometime on Friday. ( I was in a meeting all morning so don't know quite when or how long, but did miss out on my sister phoning me to tell me she had secretly got married.
Oh no! Bad luck missing out on THAT call!! Whats the chances???!
I have answered this now. Turns out that current phone has standard sim and new one has micro sim - DOH!!!
So no matter what I do it aint going to work! Guess what? They can replace the standard with a micro, but DH has to call them!!!
Phone is totally network free, so I guess we will go with GiffGaff and another number for a few days then he can ring for the PAC code on, let me see, boxing day?????
THanks, glad to hear GIff Gaff good.