You may write "stable" v5. We all agree that v5 (even the latest 5.0.4) is everything but absolutly not stable. I would also suggest you to avoid using it in a production environment.

There is actually no migration/updating script available. You can migrate to v5 but you'll loose all the actual billing/invoices history of your customers (or you'll have to import them all manually). On that point, I understand why they don't release an import/upgrade script yet: This is mainly because v5 is still beta . Modifications happen everyday on its code/kernel/database structures. It would be a titans works to update the import script on each new release.