I have found a clear answer on this post (siggraph posts)
In short, you loose perpetual (R21) by converting to subscription (when new version R22 arrives)
What you can do is not converting from perpetual R21, but going for a new subscription R22 and onwards. This way you do not loose your perpetual R21 (and not having to pay anymore), and from R22 and onwards you go with the subscription model.
I my case, my MSA R20 expires end of august, the best choice is renew (for the last time my MSA) and get perpetual R21.
From R22 (august 2020) i will take a new subscription (R22) or take a conversion for two year subscription (R22 and R23) with a 20% discount (but loose my R21 perpetual. I then still have my R20 perpetual.