Лошадей на переправе не меняют

When you are in the middle of fulfilling some task, you should not make drastic changes

Cf: Don't change horses in the middle of the stream (Am., Br.). Don't swap horses crossing a stream (in midstream, in the middle of the road, in the middle of the stream) (Am.). Don't swap horses while crossing the stream (Br.). Never change horses in midstream (Br.)