You’ve opened a new account so it will have a new number and a new card. Nothings probably “gone” it’s all still where it was, in your existing account. You now have 2, one for business and one for personal. I can’t think of any bank where they would warn you that your new account would have a different number to your current one - you can’t have two accounts with the same number.
I guess, from the context, you thought you were doing some sort of account conversion? I’ve never heard of that so I don’t know if it’s possible with any bank but it’s certainly not ‘a thing’ with Monzo.