Data element on published BP should be linked - what are our options to fix this?

On Version 23.2 (Cloud), uDesigner:

Working on a modification to a BP that has been published, we noticed that a data element we want to use was already in place in a hidden block. Our intent is to add this as a linked DE with a Shell Attribute Form source type, but since it's already in place on a form, it does not appear in the list of available DEs in the 'Add by Shell' list. To get this to appear in the list, the DE apparently must be removed from all forms on the BP.

If we remove the DE from the form and add it back as a linked DE, the error check message comes back with 'Data element [de_name_here] is removed from published Upper Forms'.