Microsoft 365 Account Swap

Hi,

I've had my current Hotmail account for over 25 years. Recently, I noticed that every 20 minutes, there's an attempted login to this account. It's set up as passwordless, and I've taken all possible steps to secure it — but given how frequently it's being targeted, I'm concerned it could eventually be compromised.

Because this account contains a lot of personal data, I’ve decided to create a new Microsoft account where I plan to purchase a Microsoft 365 Family subscription.

Here’s what I’d like to do:
Purchase Microsoft 365 on my new account.
Add my old account as a family member (it currently has about 1 month left of Microsoft 365).
My old account has 4 family members already — I’d like to make sure they are not affected during this transition.
My key concern:

How can I ensure that no OneDrive files or data are lost — both from my old account and from the accounts of my existing family members — when I make this switch?

Any guidance on how to make this transition smooth and secure would be greatly appreciated.

Thank you!