Portion of iRule not being processed

Sorry, this one should finally work. Your nested switch needs the -glob:

when HTTP_REQUEST {
switch -glob [string tolower [HTTP::host]] {
      "oldforum.olddomain.com" {
          Check the URI, set to lowercase
         switch -glob [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
            }
         }
      }
   }
}