Hello,
Users would NOT have to sign up again. All users with the has_wallets capability can hold balances and use the plugin.
The user transactions (and therefore balances) will be migrated when you install wallets6 by a cron task. This means that the MySQL rows will be copied into the new custom post types (wallets_tx and wallets_address post types). Users will not be able to use the plugin until the data migration finishes. How long this takes will depend on how much data must be migrated. This is very safe, even if anything goes wrong, you can always go back to using wallets5, since the migration process only copies and does not delete data. Once data migration is successfully completed, you can backup and delete the two custom MySQL tables (wp_wallets_txs and wp_wallets_adds) at your own time, to save space. All of this will be explained in detail in the release notes.
Using a subdomain is possible, but it is likely to cause more problems than it solves. If you can avoid it, it’s better to use a single-site installation.
with regards