They won’t. That is not going to happen in a launched live game. They already have Seasons where you have to make a fresh character and get fresh items if they want to do a complete overhaul of systems. There are also expansions which can serve as a “reset” to move players to a new system.
You don’t own the Battlenet account, game code, items, or anything else. The EULA covers this situation and you agreed that they can change the game, including removing items, chars, etc. at any time and don’t owe us anything.
Alterations to the Platform and Availability. Blizzard may change, modify, suspend, or discontinue any aspect of the Platform, Games, or Accounts at any time,
including removing items, or revising the effectiveness of items in an effort to balance a Game. Blizzard may also impose limits on certain features or restrict your access to parts or all of the Platform, Games, or Accounts
without notice or liability. Except as otherwise set forth herein, Blizzard does not guarantee that any particular Platform, Game, or Account, or any particular features or components thereof, will be available at all times, at any given time, or in all countries and/or geographic locations, or that Blizzard will continue to offer the Platform, Game, or Account, or all features or components thereof, for any particular length of time. Availability is subject to change at any time, although we will endeavor to use reasonable commercial efforts to provide you prior notice, unless the discontinuance arises from a matter that is beyond Blizzard’s control or causes the provision of such advance notice not to be possible or feasible.
They won’t DO that, but they could if they wanted. They usually limit it to changing how game systems work like attack speed or something. Then older items become obsolete.
Yes, access to cosmetic looks you paid real money for are not something they would take away unless the person violates the EULA contract (cheating, hacking, etc) and gets their account closed. That, or financial fraud.