RFC 4034, Section 6.2 describes the canonical form of resource records. Point 3 describes that for certain types, including NSEC in the DNS name embedded in the RDATA, the upper case letters have to be changed to lower case.
In a confusing way, RFC 6840, Section 5.1 requires that NSEC has to be taken out of this list and NSEC RDATA be kept as is.