In my opinion changing the lvl 100 helms makes no sense after 2 years. You say that "it was never meant for pvm" but melee attack works the same way with average damage, so it kinda was meant for pvm.
If you wanted to change the helms to their "real purpose" you could have done it since 2 years ago, without players investing resources into an item that will eventually be "erased".
I believe the change in lvl 100 helms comes on the background of the new sash system, with the idea to balance damage in pvm because sash system boosts it already. This can be balanced simply by making the sash system exclusively pvp.
Why boost pvm bonus with sash and nerf it with helms? What's the point of this?
Adding damage bonus somewhere and substracting elsewhere is not the way to go if it involves damaging players' time and money spent on this server.
If you do it on systems that were created before your arrival, maybe this is justified but in this case the lvl 100 helms were implemented by you and you had all the time in the world to "fix" them if you truly wanted to.
If the idea behind these changes is to make players spend more time and money on items, then it's again wrong. The proper way is to add new features on item mall/ milleage mall and decrease drop rates at certain items that are available via item mall. For example, you could adjust the drop rates of changers and other items that lost value in order to fix market gaps rather than changing items that people already made on a specific purpose.
With these changes, you only make people frustrated the same way you would be frustrated if someone would take away your 1 month work over an update.
You should also take into consideration that players are of different ages, social and cultural background and sometimes overreact not because they have something against you but rather because they feel you defy them.
I think it's your responsability to be wiser than us and to find solutions with us to come to a compromise and not be so harsh when players criticize. It's your right to delete offensive comments but try not to generalize and try to look from the player's perspective aswell. After all, our goals should not interfere with each other.