It would require NOOBS to be able to shuffle / move / resize the partitions, which is well beyond the scope of what NOOBS does currently.

For example if you had
/dev/mmcblk0p5 Raspbian boot
/dev/mmcblk0p6 Raspbian root
/dev/mmcblk0p7 Retropie boot
/dev/mmcblk0p8 Retropie root
then what would NOOBS be expected to do if you deleted Raspbian?

You can do what you're asking for manuallly using something like GParted on a regular Linux PC (and then edit the files on the /dev/mmcblk0p3 SETTINGS partition on the SD card to reflect the new partition numbers and available OSes), or you should be able to make use of https://github.com/raspberrypi/noobs#how-to-create-a-custom-os-version to backup your current OS and then 're-install' it as the only OS.

https://github.com/raspberrypi/noobs/issues/238

另外还可以参考这里：
https://raspberrypi.stackexchange.com/questions/59154/re-configure-raspberry-pi-boot-menu/59899#59899