First of all I am not sure that the Grub on 12.04.2 is the same as the Grub in 12.10 and 13.04. I think that 12.04.2 is still Using Grub 1.99. I know that 12.10 and 13.04 are using Grub 2.0. What is the difference?
For a start with Grub 2.0 we get an Advanced Options for Ubuntu sub-menu that contains all those previous Linux kernels and recovery mode. This is a much neater way of presenting the boot options in my opinion. Do you see an Advanced Options submenu? If not then you still have Grub 1.99.
Any install by default will install its version of Grub to the MBR. Thus overwriting the version of Grub that is already there. The way out if this is to boot into 12.04.2 and run
But, believe me you will not like the resulting mess that you will see. Grub 1.99 does not understand the Grub 2.0 configuration files and you will see duplicate and even triplicate menu entries.
sudo grub-install /dev/sda
You need to create a separate 15-20GB partition to install 13.04 in. Yes, Ubuntu can install in about 5GB but during development we get a lot of downloads and old libraries are not necessarily removed. I am running 13.04 on a 13.3Gb partition and 9.8Gb of it is used up.
When you install you choose the Do Something Else option and you create the new partition. You select that partition as the location do install Ubuntu by selecting the partition and clicking 'Change.' You select Ext4 as the file system and mark it for format; give it a mount point of / and that is about it.
There is nothing 13.04 about installing 13.04. It is all very installing Ubuntu. We follow the same methods to install 13.04 as others do when installing 12.04 and 12.10.
You should read through this: