The AT+WHCNF Hardware Configuration command description says,
I presume, then, that this would be the way to do it - although I haven’t tried it yet…
Actually, it sounds like this is another one of those things for which the Open-AT API provides no support - you have to do it manually with AT commands. 
See: viewtopic.php?f=11&t=2031&p=7587&hilit=messing+about#p7587