I posted another version of the service with the field renamed and it is no longer blanking that field out when I take it offline. Initially I thought that perhaps Area was a no-no for a field name but since it worked fine when I was online I assumed that wasn't the problem.
I will keep testing this but for now I have another question. How can I keep the attachments I have in the current AGOL service? I assumed I could copy my data from AGOL back into Desktop, fix the field name and post it back out which all worked fine for the point data and attributes but it does not keep any of my attachments.