When you change os-type, some hardware settings are changed accordingly (i.e. storage-controller, disk-type, network-controller, etc.), because some are supported in particular OS. I'm not sure what your particular case is about, but very probably your boot-loader can not suddenly find other disk-type/controller it had been configured for during OS-installation.
I see three options:
1. do not change anything and let it be "other linux 64bit". It really does not make big difference. That "other linux" is something like "the most compatible hw-settings any linux-like OS should support".
2. change OS-type as you wish, then boot with installation/recovery-cd, but do not install OS again, just mount partitions and modify your boot-loader settings to reflect new disk/partition-path.
3. change OS-type as you wish, but edit VM-settings afterwards so that it keeps the old disk-type and controller (the same it had as "Other Linux 64bit")