Try this.
if { ([HTTP::host] equals "webservice.xxx.xx" ) and ([HTTP::uri] starts_with "/webservice") } {
HTTP::header replace Host "ihstestweb.xxx.xx:1234"
}
The uri is the same so why do you neeed to change it?
You can try this to see if it change the uri as well, but I am not sure if you need this step. Something to try tho. Not sure about the syntax but just give you some ideas.
if { ([HTTP::host] equals "webservice.xxx.xx" ) and ([HTTP::uri] starts_with "/webservice") } {
HTTP::header replace Host "ihstestweb.xxx.xx:1234"
HTTP::header replace uri "/webservice"
}