Proxy between UNIX socket with socat

I used socal to create a proxy server between 2 UNIX sockets: /var/a.sock , /var/b.sock client -> /var/a.sock -> MY_COMMAND -> /var/b.sock -> MY_COMMAND -> /var/a.sock ->client. The following commands works fine just for client -> /var/a.sock -> /var/b.sock -> /var/a.sock -> client. socat UNIX-LISTEN:/var/a.sock,fork 'UNIX-CONNECT:/var/b.sock I need one more thing. To replace AAAA with BBBB before send How can I do that please ?