The title really says it all, but just in case, here's some context:
Each time you change your configuration in NixOS, you need to run nixos rebuild to create a new boot image, which will be listed in Grub when you start the computer. A new configuration might require a new kernel. If it does, and you build it, will your old configurations continue to work?
In Ubuntu it appears that one can indeed host multiple kernels on the same machine. And I read somewhere the linux kernel can be pretty small, like 60 MB. Those two facts lead me to expect NixOS will retain the old kernels. But I haven't found anything online that really makes that explicit.
I am currently building a configuration that uses Musnix. If you ask for it, Musnix will build you a realtime kernel. I'm currently building such a new configuration, and hoping I'll still be able to boot my computer after it. I worry because GIthub user @magnetophon, who is involved in Musnix's development, said the Musnix realtime kernel is borken.
This is one of the cool features of NixOS. When you run nixos-rebuild boot (or nixos-rebuild switch too for that matter), it will create new boot entries alongside the old ones. These entries have the right kernel and system configuration in them. So if your experimental kernel doesn't work, you can just reboot and start a previous version of your system, knowing that it will work, even if your kernel also came with userland changes.
The nixos-rebuild command is documented here in the NixOS manual: https://nixos.org/nixos/manual/#sec-changing-config