Thank you for your response, but as usual some new knowledge creates new questions.

HF20 won't take place until 17 of the top witnesses run the new version.

So this is done automatically? When the number of witnesses who upgraded to HF20 will reach 17, there will be loud ping and now suddenly steem blockchain is running HF20? :-)
I mean who is confirming blocks with HF19 code when almost 17 witnesses are already running newer version of code? Are there two versions of code running simultaneously on each node or the HF is somehow compatible to the previous HF (I do not think it is latter)?