Some of the Armada paperbacks were considerably abridged compared to the Chambers hardbacks (Collins wasn't the hardback publisher). This list
from FOCS shows the extent of the cuts. In general, the later Armada titles (from the late 1980s) were uncut although you'll see that some of the earlier titles were too.
Those which are described as having "very major" cuts include:
Head Girl - one chapter missing and other large cuts including our introduction to The Stuffer and Maria.
Exile - one chapter missing after the group arrive in Switzerland and numerous other cuts.
Three Go - a chapter and a whole storyline missing (Gillian Linton meets her future husband).
Coming of Age - a chapter missing which includes Miss Bubb
I can't remember the details of the other books in this category and no longer have the Armada versions but I'm someone will be able to help.
I'd describe Highland Twins as very major too as it has at least a chapter missing.
In terms of "updating", later Armada versions take out all references to staff smoking and the forbidden book Jennifer reads in Wrong is changed from Gone with the Wind to Forever Amber.