Hello,
If you purchased it from Best Buy, then all billing matters do go through Best Buy, not Microsoft. Microsoft explains it here:
https://support.microsoft.com/en-us/account-billing/manage-your-microsoft-365-subscription-purchased-through-a-third-party-44749f45-4c0b-5d41-a73f-7634b8d2a763
So the adivce you got is correct. You would need to let it expire, make sure you don't have auto-renewal on with Best Buy. Then sign up with Microsoft directly.
As long as you use the same Microsoft account to do so, you shouldn't experience data loss. Make sure you time it correctly, so you are not leaving a big time gap between expiration and signing up with Microsoft.