I encountered an issue when trying to publish changes to an existing workflow. Even though there were no errors when I checked for errors, I received the following message:
The form <form name>.xsn has been customized using InfoPath and may no longer be compatible with this workflow due to field changes. To continue using the existing form, you must first update it using InfoPath. If you no longer wish to keep your customizations, you can delete the form and a new one will be generated when the workflow is published.
Since this was referring to the initiation form that I had not customized I saved the workflow, deleted the initiation form, and published the workflow. This fixed the issue since a new initiation form was created.
If you did make customizations to the form, first try to open the form in InfoPath and republish the form to see if that fixes the issue.