Looking to switch to Plusnet mobile, but old contract still valid

[#1]

I have a month left on my Three contract, it ends on the 19th of January.  I received an email this morning offering me a £10 month deal which ends on 13th January.  Can I take this offer now, and then next month when my old contract ends, transfer my old number over to this one?

[#2]

I think you can. You just request a PAC code form your old provide when you want to move and ask Plusnet to transfer in your number, giving them the PAC code and the number you want to bring in. You can do it when you like, allowing for the fact that the PAC code will last about 30 days, at which point you would have to request another one if you still wanted to make the transfer. 

I'd read around on the forum with respect to number porting, before committing, a number of people are having issue with the experience (me being one of them). Maybe by then some of the issues will have been resolved.

[#3]

ok so I should take the contract with Plusnet a day or 2 before the offer ends, and then a week or so later, request my PAC code from Three and give this to Plusnet?  Or would it be fine to request the PAC code from Three on the same day I take the Plusnet contract?

[#4]

PAC codes are valid for 30 days and by themselves do not do anything. Asking 3 for a PAC code is just that, asking for a code. It makes no difference to 3 nor is it notice that you are leaving. 3 will still provide your service until either you tell them to cancel OR the PAC code is used by another provider (in this case Plusnet)

In the past, I've requested PAC codes multiple times as I did not use the code within the 30 days.

Also PAC codes have to be issued with 2 hours if you use request it via text or company website, but if you phone then while you wait.