Deleting Fields in Marketo

Hi community - hope you can help here.  We are in the process of cleaning out old fields within our instance (or should I say, hiding them and blocking updates to them).  I understand that in order to do so, they cannot be in use.  We were removing them from programs, but noticed that the email program, once it was sent, we can no longer remove the fields from that program.  The community suggested getting support involved to remove these on the back end.  However support came back and said that it will cause data corruption.  Their recommendation was to delete the program, but then we lose the data with that program.

Has anyone had to do this before, and can you let me know what this corruption was?  Support could not answer that for me.

Also, any recommendations would be appreciated!!!