If I upgrade to professional do all my collaborators have to as well?

[Ross Novotny]

I create and maintain several Trello boards for my consulting clients.  If I upgrade to professional, do all of my clients who collaborate on the boards have to as well?

[Iain Dooley]

@Ross Novotny They don't have to upgrade, but any Trello user who is a member of more than one of your boards in a business class team will be billable to you as the owner of the team:

https://help.trello.com/article/855-how-billing-works-with-trello-business-class