Hi,
I think teiPreFilter.xsl should be the only place you need to change, if you just want to redefine the 'date' field, and not make a new field (e.g, 'origdate') in addition to the old one. (I did the latter, and had to change lots of places.)
But your modified 'date' will not work until you've done a 'clean' reindexing.
- Øystein -