Someone more knowledgable may react soon enough, as I am not sure, but without looking online my steps would probably be (assuming you have some form of a boot menu installed):

1. Install older version
2. Reboot to boot into that kernel
3. Uninstall the 52 version while booted to 51

Have a look at:
http://wiki.maemo.org/Kernel_Power#U...g_kernel-power