I have template for WIN hosts with simple low level discovery rule from Zdocs, no actions, I just link this template to host manually.

Lets say I have Template_WIN_old with no LLD and items like "Free space on C:" and new Template_WIN_new with LLD for Disks stats :

1. Lets unlink "Template_WIN_old" from WIN host, just unlink becouse we want to save data.
2. Then link new "Template_WIN_new"
3. Now discovery rule can not add item item like "Free space C:"(key) on host becouse this already exists (better to fix I guess).
4. BUT its OK to add triggers
5. Then every 3600(discovery period) secs we have + more triggers, for 1 day I have +1500 triggers with 2 triggers configured in discovery rule (with 120 secs period of discovery just for test)