Here are the things you need to keep in mind when changing mobile phone networks on your pay monthly contract:
You may want to retain your old mobile phone and use it on the new network, this way you dont have to sign up for higher priceplans. Just stick in the new sim card into a new handset and start using.
Before switching mobile phone networks check whether your phone is unlocked, if it is not, ask your network to unlock it for free. Because you are movig to another network, and because you have finished your contract they need to unlock it for you. You could unlock your mobile phone either from your network directly or from third party companies. If you do unocking with your network then normally you wait a minimum term and pay a fee, unless your contract is finished. You can change mobile network, keep number and improve your existing deal.
However, before you do that you need to identify best mobile phone network coverage or in other words best mobile phone coverage in the uk. Certainly you may ask a question: which mobile phone network has the best coverage? Coverage depends on the relative location of antennas and or receivers of signal for networks. The closer you are the better the signal is.
Also you can transfer mobile number to another network when migrating from one provider to another. Ask for PAC (Porting Authorization Code) code from your network and tell your new network provider to use this code to transfer your number.
Alternatively, you could buy a sim free phone which is unlocked already.