Augment old or add fresh Content?
At site redesign, the question does arise for the content provider of an already exisiting site?
A simple answer would be : “Fresh content is good, especially when it is good content”. But Old Content is part of your history in SE index; it acts as some sort of foundation. Build on it. Add some new on top of the old. Augment!
In conclusion, keeping the old content in place and adding new one on a regular basis would leverage legacy and freshness with both your visitors and the search engines.