Thanks for the peek under the hood, Tim.
Assuming the change will require a hardfork, then you will need at least 17/21 of the witnesses (20 primary plus one backup = 21) to vote "yes" for the change.
You perked my interest in knowing how the 21st witness is choosen to participate in the vote?
If at the time of the hardfork, 17 of the 21 witnesses in that block are voting yes then the HF is approved. If the backup is voting no and there are only 16 votes then it can still be approved in following rounds if a new backup is chosen that votes yes.
Ok, so it is all software version triggered? For example, if 17 of the witnesses install the new fork that is how consensus is reached? Sort of like the version triggers you mentioned in the post?
Yes