Installing older firmware over newer firmware, is not supported by Kobo and nothing about it is guaranteed. You can't be sure that everything was removed and replaced correctly. Usually, it is fine to go back a few versions, but, if there are any issues, then you need to consider the full factory reset. And there are some firmware versions that are basically cut-off points that restoring earlier firmware won't work.
You can try it, but, if there are any problems, you need to do a factory reset.
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