Indeed, as always, keep copies of your correspondance (preferably by email) and addendize them into each of your reports.  This way, you have the client's clear request as to the nature and exact parameters of the work they are ordering from you.  It's not about dating-down from someone elses work, but about completing a specific abstracting project. 

After all, we don't change (i.e. lower) our standards for different types of research.  We do the same comprehensive work each and every time, whether someone requests one year, ten years, forty years or more;  report everything, miss nothing.