Hmmm…
Another approach would be to disable the confirmation question by temporarily commenting out lines 215-220 in file /path/to/nextcloud/core/Command/Db/ConvertType.php (https://github.com/nextcloud/server/blob/v14.0.3/core/Command/Db/ConvertType.php#L215-L220). Just add a double slash // at the beginning of those lines. Maybe that is easier than fiddling around with pipes and stuff.
Don’t forget to do a backup of the sqlite-database (owncloud.db in the data-folder) and the config.php before.