I was in the same situation as the OP 2 years ago and because of the migration nightmare I kept both accounts, new subscriptions would occur in the new account while old ones would stay in the legacy account.

Stripe has many issues, the missing migration is one. In my case there is alos the fact that there is no "company name" field and there is no way to issue an invoice correction for subscriptions... This makes it problematic for B2B use cases. I have alrady contacted them multiple times to ask them to add these very basic features but to no avail.

Had I known this before I would probably not have used them.