Only thing is be sure to check the policy of 3 and the PayGo sim's network, wrt porting.
Some networks wont let you port in and out in the same month. So if you port to a PayGo you may have to wait 30 days before they will give you a PAC to port away.
And I don't know 3's policy, but I know Vodafone for example won't allow you to Port a number in after the contract has started. O2 I believe will only allow you to port in within 30 days of signup.
You don't want to have your number stuck on the PayGo sim and not be able to get it on your contract before 3's conditions for porting in expire