There's an existing thread on this topic from a few weeks ago --
Delta Adds 'S' as New Comfort+ Booking Code
Complimentary upgrades to C+ will continue to rebook into "SU" class (not simply 'S') which is exactly the way it works right now.
There are 5 domestic FC classes (J/C/D/I/Z) and DL does not distinguish between them for SDC, so I see little reason for concern simply because they are going from one to two C+ fare classes. As noted, C+ fares all have an underlying coach class and they could have already been using availability in that underlying coach class for SDC if they really wanted to distinguish between C+ fares.
Looking at the fare filings, it's clear that this gives DL the ability to price C+ somewhat independently of coach bucket availability. If coach is wide open, but C+ is getting filled, they can keep open V bucket availability for coach fares, but zero out the S bucket for C+ fares. This means that only the pricier W fares with underlying V can be purchased for C+. For domestic flights, W fares look to typically be $30 more than S fares which in turn are typically $30 more than the main cabin coach fares with matching filings.