I think CEM differs mostly in the number of question types, reduced time in which to answer and, in our area, the inclusion of a comprehension exercise. I definitely agree with the former post that it is still tutorable but that tutoring needs to be more intensive to cover all the different question types.
However, we had been tutoring since year 3 and slowly building up to GL since the end of year 4 but it was only six months before the test that word spread that the school had changed exam board. Even our tutor seemed to be in the dark about it. We were already about of a third of the way through the BOFA online tests (which are actually GL focused but did help on the day) and DS1 complete them before test day. His tutor started sending small pieces of literacy work home including comp exercises, clozes (fill in the missing words) synonyms, antonyms, etc. As you are currently tested on English some of this may be familiar to you but it was a shock to us how much extra work there was to get through! We increased the tutor to two sessions a week to dedicate a whole lesson to practicing the literacy elements (bizarrely, in my opinion, referred to as VR) On the day we were very nervous but I think the "tutor proof" hype had gotten into our heads.
I suppose what I am trying to convey is that the short answer to the original post would be that it is different and it is a lot more intense but as long as you are well prepared for GL the change to CEM is not as scary as it seems.
Hope that helps
Thank you that had helped, we are learning what the GL papers are like back to front with older DD right now and she is being sent stuff home from tutor so I get to see it all. Glad to know it's not a million years away from CEM, I believe it will be CEM by the time younger DD sits it and I like to be prepared.