I have a Gold membership that will switch over to a Premium trial tomorrow. The email I received says
Does that mean that current board members who may be on multiple boards in a Premium Workspace will be grandfathered in and still have access? Or does it mean that they will keep access, but I'll have to pay for them as additional members?
When personal boards are moved into a workspace, the guests on those boards will still be able to access the boards as long as they stay open.
If you then upgrade that workspace to a subscription, you will be charged for all billable users on that workspace. Billable users are members of the workspace and multi-board guests (users that are on two or more boards in the workspace). Single-board guests are not considered billable users.
If you want more information pertaining to a specific workspace of your's, you can reach out to Trello Support at https://trello.com/contact