I believe that is a different issue.
You need no PAC code unless you are changing service supplier, which isn't something I deduced from the OP. I believe the issue here is simply getting a different sized SIM to suit the new handset. (It occurs to me one can get kits to trim larger SIMs to smaller ones anyway. Whether it's worth it is a different discussion.)
A handset only needs to be unlocked if you got it subsidised from, and thus locked into one supplier, and wish to use it to connect to a different service supplier's network. Again I didn't see that as being the issue.