Is it possible to change a lookup field after it has been added?

Here's my situation.  Form B has a lookup field that points to Form A.  I now want to insert a subform in Form A of all the Form B's.  When the lookup field in B was created it was as a new relation, not an existing relation due to the subform in A.  So the Form B's don't appear in the subform.  Can I get that lookup field to change to the existing relation of the subform?

I can almost accomplish what I want by adding a second lookup field in B using the exisiting relation of the subform, and then copy the ID's stored in the original lookup.  That works.  I can't delete the original lookup, however, because I have referred to it in too many other deluge scripts.  So now I have two lookups and have to maintain that.  I don't like it.