From the next CTAN update, the semantics of \int_to_base:nn will be altered. It will in future return lowercase letters when using bases > 10 to represent numerals beyond 9. This change is accompanied by the introduction of \int_to_Base:nn, which returns upper case letters under the same conditions.
This change has been introduced as both lower and upper case letters may be required, dependent on the use case. See also announcement concerning \int_to_hex:n/\int_to_Hex:n.