Currently my dad has a pay as u go sim card on t mobile - he is spending about £10 a month therefore it would be better for him to go on the sim only deal for that amount and get some minutes and texts. The sim card is not registered under anyones name as it is over 10 years old and in those days there was no need. If i wanted to move him over from a pay as u go to sim only deal can this be done?
He is on t mobile.

The same goes for my mum, she is on o2 but unregistered sim card - could i request a PAC code and move her to a sim only deal on another network or even the same?

i am sure it i will be possible even if you have to phone them first to register the sim and then again to move to sim-only. I guess you might have to give them information such as frequently called/texted numbers, age of sim, top up method....., in order to verify that you are your parents.

if they call each other a lot it might be worth moving them to the same network (as you as well?) and selecting a deal that give free mins to your friends on the same newtwork.