Hello,
First I know that custom fields are a premium feature and one of my companies pays for that. I have two accounts for two companies and often use Company 2's account as it is my "main" account but it is on the free plan only.
I was customizing a board created by someone at Company 1 (paid plan) using a lot of custom fields. I realized it was viewable to the entire workspace when it should not be. I created a new workspace and moved the board to the new workspace. My custom fields all disappeared and Trello now says to upgrade to add custom fields.
I may have moved it using Company 2's free account, but why would it let me do that? So it seems I created a new workspace on my free account and that deleted the custom fields and several hours of work with no warning. Is there a way to undo this?