We need the BP on purging old accounts and contacts

We are trying to clean up our contacts and accounts and have a BP question.  Is it best to purge all old contacts?  We agree that we should clean out all the old data, but how do we transfer any important info like if the old contact was tied to an opportunity.  Or does it matter.  We have thousands and thousands of accounts to clean up and the sales reps do not have time to go in and make notes in all of the accts where we might want to save some details.