Your old files will not work with P5:

1. <TEI.2> is now <TEI> and <teiCorpus.2> is now <teiCorpus>
2. TEI elements are in the http://www.tei-c.org/P5/ namespace
3. many attribute values may have changed… Y|N is now true|false
4. … or disappeared
   • text-valued attributes to become child elements
   • lang to become xml:lang
   • id to become xml:id
   • all IDREF (target) values to become URLs
5. Your TEI extension files will almost certainly require re-writing —but there will be substantial payoffs