The easy way is to install auc (opkg update ; opkg install auc) and let it do the work for you. Both auc and LuCI Attended Sysupgrade have internal logic to detect the current file system format and select the proper installer to match (in addition to keeping around all your user-installed packages).
Alternatively, you can download my package scanner script (see The OpenWrt Firmware Selector - #621 by efahl) and it can tell you all about your current installation, to help you do things manually if you so wish.