[apefish]

I think there are many "fair" arrangements with each having different benefits for the varying parties.
For example...

A) the horse works off his makeup with A and then is autofolded in the new stable. in the meantime staker A continues original staking and A+B can still do business on others.

or

B) new stable is formed immediately with original makeup still pending: priorities are 1) makeup to new stable owners, 2) profit share to new stable owners, 3) makeup to original owner A, 4) horse. (this requires potentially some reworking of percentages until the original makeup is done)

the horse won't like this one because he's going to get an immediate lesser percent, but it's probably better than him being cut off. You could also lower his percentage to go to the original makeup once new stable gets their cut until he is out of stable A's makeup.