I think it was valid only for users with last "before-subscription" version R21. You could went to subscribtion with conversion from last version before go to subscription. All versions before R21 (R20 and older) had serials instead of online licencies. If user own R20, can not "continue" with subscription since R20 was last version without online activation, you could start new subscription (as totaly new user without any version of c4d)
So, if R21 perpetual licence owner went for subscription (there was time limited offer with some benefits), get new S22, but his R21 permanent licence become unusable and was converted to S22 subcription model...
Not sure how it´s currently. If R23 permanent licence holder had offer to switch to R24 subs (but lost his R23 perman. licence) or was not posible this kind of offer in the past. (since currently Last version is R25)