Change account email.
I found my answer here: https://support.guildwars.com/home (Old GuildWars site, but contains the information to changing the associated email account between both.)
I’m not sure what that article says, but if you found information on changing the email address associated with logging into the forums, you found the information needed to change the email associated with logging into the game. Both use the exact same information.
Only the CS Team can change the email address associated with your game account/forum, and the link is at the Top of Page – Support – Submit a Request.
Good luck.