MicrosoftOne more observation to add: even if you jump through all the hoops necessary to apply colour highlight to text in modern comments (via the “start” ribbon), it now disappears when viewed from the mobile version of Word (Android) – I must admit I am not quite sure if that at least displayed highlights before, but I somehow think so and it strikes me as missing now...
About the different build versions and who gets to “enjoy” the redesign: having followed all of your reports so far, it seems evident to me that the switch is not simply linked to the version or build of the program or the development channel you are in. If people have had success with reverting to traditional comments, but needed to roll back to different previous versions until the reversal was eventually triggered and things “snapped back”, this can only mean that the code for modern comments has been implemented quite a while back (@TonoS mentions having had to return all the way back to 13231.20110 to escape). It seems that MS has simply now arrived at randomly activating users in the current channel in a sample-sized trickling rollout, but this must be a change that is triggered from the server-side for each running instance of the program individually (my second PC, same version, same 365 account, has not been switched despite working on the same online document).
I also suspect that this is a change that should be reversible in both directions within the same instance and version of Word. I noticed that my PC already contaminated by modern comments opened at least one older document in the old design (although I do not remember which document and cannot currently reproduce this – not sure if it was in safe or compatibility mode or whatever). The point is that I am fairly sure that this is something that in principle could be toggled between both modes, at least for the time being, but MS won’t let you (although or rather because I suspect that they intend to entirely remove the old code eventually. They'll just sit this one out until we nay-sayers get tired and grudgingly accept our fate).
Maybe what we need are not beta-tester but hackers, which makes me wonder why I am still paying for a legitimate licence...
