I couldn't find triple Hollowtech II, 50/39/30. As a matter of facts, triple 50/39/30 of any kind proves to be rare. I found an offer for Shimano Claris / Octalink, at roughly 40 EUR, which looks pretty much like the crankset I want to replace (with the difference that the actual Prowheel is square, not Octalink).

is it possible to switch the old Prowheel chain rings with the new Shimano Claris chain rings, on the old crank arm/spider of Prowheel Ounce? The actual Prowheel states "130 mm BCD" on the large ring; for Shimano Claris - I do not know, but it looks very close to Prowheel: