Do you have access to MySQL? I had to migrate 1,000+ users from a legacy website.
In the wp_usermeta table there are several rows for each user one of which is wp_capabilities
I added a new user via admin to get the default settings for a subscriber, and the entry is a:1:{s:10:”subscriber”;s:1:”1″;}
I then added/updated the values for all the users to this to make them all subscribers.
You’ll need a bit of SQL know-how…
And… backup your database before doing anything via MySQL!!!!