This is a stack that will fix the LiveCode repo to point to their new username. While we are poking around it also fixes the submodule remotes to point to your forks of them.

RepoFixer.livecodescript

script "RepoFixer"
on preOpenCard
   create button "Fix Repo"
   set the rect of it to 10,10,100,33
   create field "Username"
   set the rect of it to 10,40,100,61
   set the visible of me to true
end preOpenCard

on mouseUp
   if the short name of the target is "Fix Repo" then
      put field "Username" into theUsername
      if theUsername is empty then
         answer "If you don't enter your github username we can't fix the submodules to point origin to your fork." with "Continue and live to regret it" or "Cancel"
         if it is "Cancel" then
            exit mouseUp
         end if
      end if
      
      answer folder "Choose LiveCode repo"
      put it into theRepo
      if theRepo is not empty then
         put the folder into theOldFolder
         set the folder to theRepo
         -- remotes for main repo
         put shell("git remote show") into theRemotes
         repeat for each line theRemote in theRemotes
            put shell("git ls-remote --get-url" && theRemote) into theRemoteURL
            if theRemoteURL contains "runrev/" then
               replace "runrev/" with "livecode/" in theRemoteURL
               put shell("git remote set-url" && theRemote && theRemoteURL) & return after msg
            end if
         end repeat
         
         -- remotes for submodules
         repeat for each word theSubmodule in "ide thirdparty"
            set the folder to theRepo & slash & theSubmodule
            put shell("git remote show") into theRemotes
            put the number of lines of theRemotes is 1 and line 1 of theRemotes is "origin" into setUpstream
            repeat for each line theRemote in theRemotes
               put shell("git ls-remote --get-url" && theRemote) into theRemoteURL
               if theRemoteURL contains "runrev/" then
                  if setUpstream then
                     put shell("git remote rename origin upstream")
                     replace "runrev/" with "livecode/" in theRemoteURL
                     put shell("git remote set-url upstream" && theRemoteURL) & return after msg
                     replace "livecode/" with theUsername&"/" in theRemoteURL
                     put shell("git remote add origin" && theRemoteURL) & return after msg
                  else
                     replace "runrev/" with "livecode/" in theRemoteURL
                     put shell("git remote set-url" && theRemote && theRemoteURL) & return after msg
                  end if
               end if
            end repeat
         end repeat
         set the folder to theOldFolder
      end if
   end if
end mouseUp