Slitherine

In the start of the first Scenario is stated, that the exp cap has been reduced to 325.

But all my existing units seem not to be affected by this. They still have exp 525 and Str 15.

Is this only for new units?

As I understand it, units which have an experience level below 325 cannot gain more experience than this limit, but units which start out above this level will keep their experience (although I think they can't gain any more, so if you start with a 4-star unit it will stay at 4 stars and not gain more experience).

That's correct. If you have a unit with experience above the 325 cap, that unit can no longer gain experience past the number it is at. It can, however, lose experience if given normal replacements. This was mostly done to show the severe lack of trained men to fight in the Wehrmacht at this late stage of the war. However if you're successful enough in fighting in the Battle of the Bulge, it is possible to reverse the fortunes of war and get this experience cap lifted...