Last year went from a S3 to a S4 and I am with EE. I actually messed it up, but had a result. I removed the plan (which cancelled it) then tried to activate a new plan - 2 days in it wasn't activated so I called EE, they said there was an issue and was fixed straight away. Additionally, as it is a 30 day contract and I cancelled the other one, all new contracts came with 2 months free trial 🙂
I would call customer services for your provider to see the best way to do it