I agree.
- Login to router via SSH
- cd /tmp
- wget http://foo.linksys.net/foo/foo-fw.bin
- sysupgrade -f foo-fw.bin
I cannot speak from experience. As, out of all the original Linksys WRT-series routers I've owned, I've never reverted back to stock. Also please be advised that the original firmware is vulnerable due to its age and therefore, unpatched state.