I'm using this rewriting rule to change login names into friendly names, but if mail comes in for an upper case login name, it doesn't get rewritten.

*@my.domain      ${lookup{$1}dbm{/usr/lib/exim/longforms}\
$value}fail}@my.domain bcfrtFT

The longforms database has entries of the form:

ano23: A.N.Other

Replace $1 in your rule by ${lc:$1} to force the local part to lower case before it is used as a lookup key.