You can switch your Dropbox account in:

rekordbox for Mac/Windows: [LIBRARY SYNC] tab on My Page

rekordbox for iOS/Android: [Settings] > [Account Information] > [Cloud Storage Services]

When you switch your Dropbox account, the changes will be applied to all versions of rekordbox for Mac/Windows/iOS/Android logged in with the same AlphaTheta account.

Please note that you won't be able to play tracks from the cloud storage of the previous Dropbox account because they're deleted from the local Dropbox folder.