It's actually quite simple.
1 - Highlight the field you want to change and 'Copy' it
2 - Delete the field and save the form
3 - Paste the field that you copied in step 1, this field will have all of the same properties as the field you copied, including field ID, change the type from Display Only to Optional, and save the form again.
All workflow that was associated with the original field is still associated with this field because it has the same field ID, you will want to ensure that it's in the proper views, and has the proper alignment with other fields and such, but you should be done.
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Key is Managing the Same DB ID for new field. :-)
Guru LJ has already Explained the whole Algorithm...