Do all mobile phone companys do this??! please help?

okay- im on the Three network on contract...... basically i want to go from a mix and match plan to a texter plan... but they told me numerous times that i cant unless i do an early upgrade. i can change how much of my mix and match i get but cant change onto a whole other plan (texter plan). ive heard that if u go to cancel a contract they will always give u wat u want. is this true... cos i sed right ill cancel it and he sed okay it will be �***.**..... but they obv thought i was bluffing... problem is i dont wanna pay 600quid to cancel it... but dont wanna take that chance wot shud i do?!