I would not say the domain will go back to "zero" just because the content changes, but if there are links to certain pages that are lost, then those links will go away after a while. You have to realize that sites are rank based on many different things and not just content.
But if you used to have a site, then parked the domain, and then had a site again, I don't think you will have much benefit from the old site, except for the length of time the domain has had content on it, and any links that were gained to the old site, as long as they persist.
This is going to vary depending on the buyer. For an investor that is focused more on the name and someday selling it, I would rank them as age, backlinks, traffic.
For a domainer that is more focused on parking income, it would be traffic, and then backlinks/PR that will indicate the traffic will continue.
