edit a request variable after being set?

HTTP::uri [HTTP::uri]/new/uri

Something like the above in the iRule will append "/new/uri" to the existing URI. Is that what you are seeking ? Basically, with the [ ] brackets, you are reading the variable and without the [ ] brackets, you are writing to it.

Added as answer so i can format

Here is the irule, i have *** where the functionality needs to go.

    when HTTP_REQUEST { 

    set Original_request [HTTP::request] 
    HTTP::uri "/newURI[HTTP::uri]"
     Send the out-of-band validation query to the MicroService. 
        log local0. "Calling new service"
    pool new_service_pool
  }
}
} 
when HTTP_RESPONSE { 

    if {[HTTP::status] eq 302 } {
    pool 2nd_pool 
    ***** Change the $Original_request URI ****
    HTTP::retry $Original_request 

    } else {
      HTTP::retry $Original_request 
      log local0. "No Re-direct - Carry on as we were" 
    }
    }