Can I convert a private BahnCard to a BahnCard Business?

No, a direct conversion from a private BahnCard to a BahnCard Business is not possible.

If you wish to use a BahnCard Business in the future, please cancel your private BahnCard in due time – at least 4 weeks before it expires. After the end of the validity period, you have the option to purchase the new card as a BahnCard Business via a business DB customer account.