Portion of iRule not being processed

So the query string is more than just tid=..

Try changing your switch to this:

   switch -glob [string tolower [HTTP::host]] {
      "oldforum.olddomain.com" {
          Check the URI, set to lowercase
         switch [string tolower [HTTP::query]] {
            "tid=*" {
               HTTP::redirect "https://newforum.newdomain.com/default.aspx?g=posts&t=[URI::query [HTTP::uri] tid]"
               return
            }
            "fid=*" {
               HTTP::redirect "https://newforum.newdomain.com/default.aspx?g=topics&f=[URI::query [HTTP::uri] fid]"
               return
            }
            "c=*" {
               HTTP::redirect "https://newforum.newdomain.com/default.aspx?g=forum&c=[URI::query [HTTP::uri] c]"
               return
            }
         }
      }