In this case, as long as:
a) You have sufficient bandwidth supplied by active "STM-*" bandwidth packs to satisfy their vTM estate, and
b) those bandwidth packs have expiry dates sufficient to allow you to cross-grade before the bandwidth packs expire,
c) You have a new "STDBASE" license with sufficient new-SKU based bandwidth packs to allow the cross-grade to take place.
You will be able to install the new "STDBASE" license and all the associated new-SKU based bandwidth packs. The new "STDBASE" license and the associated bandwidth packs will show as "Active" in the Catalogs > Licenses page.
Note that the old "STM-*" bandwidth packs listed on the same page will continue to show as "Active" (unless they have expired). This is important, as this means that existing instances using Feature Packs based on "STM-*" SKUs will continue to draw bandwidth supplied by these bandwidth packs.
Once the "STDBASE" license and the associated new-SKU based bandwidth packs are installed, you will be able to create Feature Packs based on the new ENT-ADVANCED, ENT-ENTERPRISE and ENT-WAFPROXY SKUs, and apply them to their existing estate of vTMs, replacing the use of Feature Packs based on "STM-*" SKUs.