Q. Can I downgrade a plugin or theme?
A. Yes. You’ll be able to do it, but whether or not the system works afterwards depends on the plugin or theme.
Some packages apply database updates between versions and don’t expect you to downgrade, so their database changes may not work. There may also be changes to other values that are stored in the database; options, automatically generated content. etc..
A previous version may be incompatible with other plugins, themes or the version of WordPress or PHP that you’re running.