While I have seen some discussions on the utilization of an older CLR 2.0 component within a newer CLR 4.0 application, how would one handle the opposite case?

For example, if one had a legacy app in .Net 2.0, and wanted to take advantage of a newer business logic that took full advantage of the 4.0 version of the CLR, how would one go about referencing the newer component within the legacy application and accessing it's methods? And would the same method work if the legacy app was in .Net 1.1?

(Addendum For clarity)

While an upgrade is ideal, there still exist edge cases where this may be a technical or political limitation.