Change field data input message

I’ve been tasked with changing the data on inbound messages. For example in every inbound message if the PID 7 (sex) is I it should be changed to a U and the message sent on. I’m using the PID 7 (sex) field here as an example, but there are going several fields involved down the road.

I’m not sure what is the best approach for this and any information or example tcl procs would be appreciated.