It may be that all you'd need to do is change the HTTP method on the second request. However, since the client is sending it as a GET, it might not be sending the body of the post request, so there might be literally no way to do what you're asking.
Unfortunately, even if it were that easy, there is no HTTP iRule command to change the HTTP method (aka HTTP verb). You could change it at the TCP layer (using TCP::collect and TCP::payload), but you'd need to know *that* you need to change it without using any HTTP commands, which may or may be possible in your situation.
If you want, you could open a case with support and have them add you to ID 224296, which is an RFE for providing an HTTP:: command to change the method. However, this won't solve your current problem.