Preexisting multiboot, overwrite Raspbmc with osmc

Help and Support
1

I have an old pi1 running a multiboot system with Raspbmc and Raspbian. (Raspbian on the SD, Raspbmc on a usb stick).
Fdisk shows:

root@raspberrypi:~# fdisk -l

Disk /dev/mmcblk0: 7948 MB, 7948206080 bytes
4 heads, 16 sectors/track, 242560 cylinders, total 15523840 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000a95e6

        Device Boot      Start         End      Blocks   Id  System
/dev/mmcblk0p1            8192     1675781      833795    e  W95 FAT16 (LBA)
/dev/mmcblk0p2         1679360    15458303     6889472   85  Linux extended
/dev/mmcblk0p3        15458304    15523839       32768   83  Linux
/dev/mmcblk0p5         1687552     1810431       61440    c  W95 FAT32 (LBA)
/dev/mmcblk0p6         1818624     9777151     3979264   83  Linux
/dev/mmcblk0p7         9781248     9945087       81920    c  W95 FAT32 (LBA)
/dev/mmcblk0p8         9953280    15454207     2750464   83  Linux

Disk /dev/sda: 7862 MB, 7862353920 bytes
255 heads, 63 sectors/track, 955 cylinders, total 15356160 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x014dc744

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *        2048    15356159     7677056    b  W95 FAT32

Now…i want to get rid of the raspbmc setup and install osmc on it instead…but i do want to keep the Raspbian setup, because it has a touch screen bootloader setup and i dont really want to reinstall the whole thing again.
How should i approach this ? I used NOOBS to install the multiboot (obviously).
Right now i am switching partitions using this script here:

-Thanks