To be clear, Kodi does not support downgrading so while "updating" to the older .tar file will downgrade the core OS and embedded Kodi app itself without any issues; there is zero support for downgrading add-ons and this may cause issues. If you see problems, you will need to stop Kodi and rename the current add-on binary folders out of the way and then restart and reinstall the add-ons you need. If you are lucky the K20 and K19 add-on settings formats are compatible; and if not you'll need to stop and rename those out of the way too.