Will this happen automatically or will the witnesses be asked to coordinate this change with a code rollout? If it could happen automatically... how?
The way it is described in the issue, it will happen automatically at the time of the HF.
When the hardfork hits, we need to modify the top 20 witness account creation fees by 30 (same effective fee) to prevent creating accounts at an extreme discount. Future witness updates will work as normal.
"we need to modify the top 20 witness account creation fees by 30 (same effective fee)"
huh?
Currently the cost to create an account is 30x the witness account creation fee. At the time of the hardfork, it will multiply all of the values by 30, but stop multiplying them by 30 to calculate the cost. It sounds confusing, but basically it will end up with the cost being exactly the same before and after the fork.