Hi Ben,
The migration from SBE to SEPC is being managed by Symantec and is currently on pause. The migration is going in waves, with MSP partners purchasing from CSP distributors being selected first.
Here is the FAQ for the migration: https://www.symantec.com/docs/TECH249314
As far as the differences between the 2 products, they are very similar in many ways, while having several distinctions.
As examples, SBE has more and better reporting options, but SEPC has more flexible options for customizing alerts.
SEPC has managed macOS installs, and supports iOS and Android, and includes MDM for Windows 10 and macOS devices.
Overall SBE is definitely the more mature product, but the support and development teams are very engaged and committed to improving SEPC.