Ah, just looking into it now. Would that process look like this? -

ReadRelationships

to see what the current relationship is - Craft a

Precondition

that ensures the above relationship still exists - Make a

WriteRelationships

request that 1) uses the precondition, 2) deletes the above relationship, and 3) writes the new relationship - If the previous step failed, you know someone else is editing it simultaneously. Repeat the whole thing.