As indicated above, the Heritage Fleet was the older RR passenger cars, mostly built in the late '40s and early-mid '50s. As I recall, the Metroliners were built '68-70, give or take, and were 'all electric' as built. Hence, no need for a conversion.
If I recall correctly, there's an FRA rule that states no in-service, common-carrier passenger cars can exceed 50 years of age. That's just one of many reasons the Heritage Fleet had to ultimately go. As I recall, the last Heritage Fleet equipped train was the Three Rivers (a Heritage 10/6 sleeper) and all the dining cars for which there was no replacement yet. No baggage car replacements were available yet, as well. The Pacific Parlor cars passed their 50th about 2006. In each case, Amtrak had to obtain a waiver (for each car? I don't know) to allow its operation beyond the mandated 50 year limit.
What's interesting is that the oldest of the Amfleet cars is well past 40, and even the Superliner Is are 'getting up there' in age. It'll be interesting to watch how Amtrak deals with those cars 'aging out'.