Isn’t the situation of preserving local file modifications covered in the Debian handbook[0]? You simply drop a config for apt to not overwrite files you’ve changed…
The problem is that during first install you can't configure the system user id range, so they'll start at 100. Later you decide to increase that to 150 for whatever reason. Even later, an apt-get upgrade upgrades a system package that owns a user in the 100-149 range. The upgrades process includes setting up the user again. User setup sees that the user exists but isn't in the system user range, this is presumably interpreted as a security risk and so fails the upgrade.
As a result the minimum system user uid is in practice not able to be increased without breaking upgrades.
As a result the minimum system user uid is in practice not able to be increased without breaking upgrades.