I have created a reusable workflow and added the site columns that I needed via the ‘Association Columns’ tab.
I have saved and published the workflow successfully.
I then realised that I needed to add more site columns, which I did in the same manner that I did previously.
I do get a warning at this point ‘Changing the association columns for this workflow can cause new instances of existing associations to fail’.
When I try to save the form I get the following error ‘Some association columns defined in this workflow are no
longer valid. This error may occur if the corresponding site columns have been deleted.
To fix the workflow, delete and recreate the affected columns’.