My estimated usage under resource utilization is now measured in MB rather than GB. Why the change? surely measuring it in GB was a better idea?

Not a clue. I dont use enough bandwidth for them to even show it. lol.
Also might be because you havent used 1GB of traffic yet. Might wait until you pass 1GB then start showing it with that.

I think that must be it, because I get

Ah, well it seems that if your current usage is still in the MB range then the estimated would be listed as MB but when it breaches the GB range, it will list the estimates as GB too. I believe this is to ensure consistency in the reporting.

I've just got one machine breach the GB range and now everything is showing up as GB.

That could have been it. It's back on GB's now.