Does this mean the partners will have to switch their reporting to "Per Core" straightaway?

The SPLA subscription program rule is if a partner was licensing a particular product/SKU at the time it was removed from the SPUR/Price List, they can continue licensing/ordering that product/SKU until the earlier of: a) they migrate to the newer version; or b) their SPLA agreement expires and is renewed.

So, current SPLA partners who have reported Windows Server and/or System Center on a "Per Processor" model under their current contract before Windows Server 2016 and System Center 2016 is generally available may continue to report under the same licensing model until their agreement expires or they migrate to the newer version.