You'll need to write a plugin that (when the condition is met) the subscription type is overwritten with what you want in the $newpost array which is then passed to function build_new_post(). Look for a hook shortly before the function within newthread.php and that's the likely hook to use.
Thanks for the help but I am not a coder so not sure what I am suppose to write! is there a code I can copy and paste? Thanks