@fmpvince: Thanks I posted it there as well.
I think one issue is that I created a lot of printing layouts in 150% zoom, and I've just noticed that actually, in layout mode 100% and 150% don't show the same thing even in FM11. The only difference is that in FM 11, preview mode 150% would display what appears on layout mode 150%. It seems that preview mode in FM12 now displays the same at 75%, 100% and 150% which wasn't the case in FM11 (on windows 7 at least).
Anyway, one thing is clear, check ALL your layouts before you put your newly converted database in production.