Here’s something to watch out for; I switched my mobile from Orange to T-mobile recently. To do this, I requested a PAC code from Orange, which lets me take my old number to the new provider. When you do this, they tell you that it’s valid for a limited period (60 days, I think). What they don’t tell you is that your contract includes a one month notice period, so you will pay the service charge for the next month regardless of when you use the PAC code.
If you’re like me, you normally request the code immediately after opening a new account. The right thing to do, if you can, is to request it a month before you want to move..