Again, Economy Comfort (KLM's C+ product), is not treated as a separate fare class product. It's just considered part of coach on KL. While C+ on DL is a separate fare class and you are put in a different fare product when upgraded to C+ on a DL fare (SU class for comp upgrades). While W is used for purchased C+, and S is used for C+ awards. KL has no equivalent fare classes for C+/Economy Comfort. Even if you find an agent to "force" a C+ seat for you on DL, there's likely no guarantee that seat assignment will "stick" since your KL fare class is still for a main cabin seat on DL, not C+.