(Have tried asking on here 2degrees Facebook support thing, just keeps asking me to connect my Facebook account, can't submit my question)

Have bought the wife a new iPhone for Christmas, requires changing to a nano-SIM.
If I get the new nano SIM card before Christmas Day, will I be able to activate it on Christmas morning?
Otherwise what's the best plan, as obviously will want to use new phone on Christmas and Boxing Day.
Do the SIM swap over the phone on the 24th as late as practical?

Cheers,
Joseph