The Linux Ship of Theseus
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pick any distro and install it.
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Then, without installing another distro over the top of it, slowly convert it into another distro by replacing package managers, installed packages, and configurations.
System must be usable and fully native to the new distro (all old packages replaced with new ones).
No flatpaks, avoid snaps where physically possible, native packages only.
Easy: pick two similar distros, such as Ubuntu and Debian or Manjaro and Arch and go from the base to the derivative.
Medium: Same as easy but go from the derivative to the base.
Hard: Pick two disparate distros like Debian and Artix and go from one to the other.
Nightmare: Make a self-compiled distro your target.
I have seen dozens of systems migrated from Gentoo to CentOS by live swapping the userspace and eventually rebooting into the new kernel. A hair raising experience to be sure.