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 \in_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.
--
Joseph Wright