Hi Rob
You definitely should include the same letters, unless you think there is something in them that could harm your PIP claim. If you have a friend or relative who knows about your daily living, then get them to write a letter.
You need to understand how different PIP is from your previous DLA claim and read through the guides here and start your claim from scratch. You're right - they won't have any paperwork on file if it was years ago.
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems