Re-write request to new endpoint and modify query string params

This might be one way to do it:

when HTTP_REQUEST {
    if { ( [HTTP::path] equals "/" ) and ( [HTTP::query] ne "" ) } {
        HTTP::uri "/[URI::query [HTTP::uri] param1]/NewEndpoint?param2=[URI::query [HTTP::uri] param2]"
    }
}

The above is dependent on the named params (param1 and param2). If the values can be different and/or you just need to move the first param, another option would be to split the query string into a list.

when HTTP_REQUEST {
    if { ( [HTTP::path] equals "/" ) and ( [HTTP::query] ne "" ) } {
        set query [split [HTTP::query] "&"]
        HTTP::uri "/[lindex [split [lindex $query 0] "="] 1]/NewEndpoint?[lindex $query 1]"
    }
}