Catching parameters in URI, rewriting.

I don't think you are far off. Your second example contains both the FID and TID parameters, but in this case you ignore the FID one. If that is how it is meant to work, simply redirect by TID if it exists. If not, redirect by FID if it exists. i.e, it looks to me like TID takes priority over FID. Would the following do?

when HTTP_REQUEST {
    if { [HTTP::query] contains "TID=" } {
        HTTP::redirect "http://newforumsite.com/default.aspx?g=posts&t=[URI::query [HTTP::uri] TID]"
        return
    }

    if { [HTTP::query] contains "FID=" } {
        HTTP::redirect "http://newforumsite.com/default.aspx?g=topics&f=[URI::query [HTTP::uri] FID]"
        return
    }

    HTTP::redirect "http://newforumsite.com/"
}