You can access the 2.4 firmware by doing a factory reset and bypassing the registration (since the registration will force an update). The registration can be bypassed using the sqlite3 technique. It may be possible to bypass the registration by backing up the .kobo directory prior to resetting the device then replacing it after the device has been updated.

Just one warning: any version of the firmware prior to 2.5 was designed for smaller and lower resolution screens. Some of the controls will be harder to press and the cover view on the home screen will be crammed into the top left corner. There were other bugs, but I forget what they were. (Needless to say, I ended up returning to 2.5.)