dabbrev-case-fold-search is a variable defined in `dabbrev.el'.
Its value is

  • This variable may be risky if used as a file-local variable.

Control whether dabbrev searches should ignore case.
A value of nil means case is significant.
A value of `case-fold-search' means case is significant
if `case-fold-search' is nil.
Any other non-nil version means case is not significant.

You can customize this variable.