It's turning out not to be as clean cut as my 3 SIM stopping working!!!
At about 8 this morning got a message from Orange saying my settings had updated on my 3 SIM (confused me a bit). I then got a message saying I had a new emails, but when I checked my 3 email I didn't have one.
I decided to put in the Orange SIM for the first time. Apart from it showing 'Orange' at the top, it didn't dial out. I rang 150 and it said my account was not setup, so I put my 3 SIM back in.
I called a friend who said they called me and left a message on Orange voicemail. So that was the message but I couldn't see it yet. I could dial out and I was still getting texts on 3 at this stage.
I continued to send texts to burn 3 mins, then noticed that 3 voicemail had stopped working. About 10 mins ago I couldn't dial out anymore and texts failed shortly after.
So I'm, almost there..! the SIM still says 3 but it's not functioning. Planet 3 doesn't work either.
So rather than a big change, it has been a gradual 'plucking' of 3 support bit by bit until I guess my 3 SIM goes dead (I'm thinking of the T800 death scene in Terminator 2 now!

).
...
I would put my Orange SIM in now to try as my 3 one is not working, but I'd rather be sure 3 is completely stopped so I know I'm not paying anymore. But then I could just see when they bill me (but could be too late). My battery went from fully charged to 10% since Sunday and it usually lasts 4 days. They must have been accessing my phone all morning!
So:
1) It's a gradually process that means your phone SIM slowly 'dies'.
2) It takes it out on your battery.
3) You get lots of notice messages that you can't do anything about.
I'll make another post when I'm over. It does seems to be worth waiting until you can't dial out, then change SIMs. I was under the assumption my Orange SIM was active on day 1, but it was not the case. Perhaps other providers activate sooner?
Anyway... Pretty boring stuff really

, but hopefully this helps someone else based from my experience.