You can use the Archive feature to convert folders to "Archives". The data remains and any active campaigns also remain active.
http://www.marketingrockstarguides.com/archiving-assets-in-marketo-1197/
What I usually recommend is to create a single Archive folder and to move old things over there every quarter and at the start of each FY.
To report on archived items, just drag in the archive folder option on the Setup of the report.