Bibtex-mode uses the variable bibtex-autokey-transcriptions to
define the default values of `bibtex-autokey-name-change-strings'
and `bibtex-autokey-titleword-change-strings'. So something like
eval-after-load will work only if it deals with all three variables.
Say, you first set bibtex-autokey-transcriptions to its new value;
then you use this to set `bibtex-autokey-name-change-strings' and
`bibtex-autokey-titleword-change-strings'.
It is probably easier to simply set bibtex-autokey-transcriptions in
your init file to its new value. Normally, emacs should load
bibtex-mode later. Then it will use your value of
bibtex-autokey-transcriptions to set the values of
`bibtex-autokey-name-change-strings' and
`bibtex-autokey-titleword-change-strings'>