There is no limit to the number of mobile phone contracts you can have so you will not need to cancel your existing contract before taking out a new one. However, if your intention is to move to a new contract while discarding your old one you may wish to cancel your contract with Orange as this will not be done automatically when you sign up for a new contract.
You can cancel your existing contract after you have purchased a new one or you can cancel it before placing your order. If you cancel your current contract before ordering a new one you may experience a period of having no phone connection. If you cancel your existing Orange contract before it has finished you may incur a termination fee.
If you wish to keep the phone number from your existing Orange contract and take it to a new phone then you should not cancel your existing contract before ordering a new one. Once your Orange contract has been cancelled the corresponding phone number will be permanently lost. You may need to supply a PAC code during the order process to transfer your phone number, and doing this will automatically cancel your existing contract (more details on mobile phone porting)