What is not documented there yet...
One thing which makes this quite cumbersome is... You can only easily do this procedure once. Well, you can rerrun this as often as you wish, but it won't have any effect.
It's because when you made changes to packages and rebuild those packages, they still have the same version number. apt-get still thinks it's the same package, hence won't update these. Does that so far make sense?
What I did in past is artificially bump the version number. Which is scripted. Btw before attempting to script such stuff, ask me, it might be already done.
Whatever package was changed...
To major version bump all packages at once...
Then the above procedure of upgrading from local repository can be repeated.
However, it can be confusing when later testing to upgrade from a remote repository. These will have sane version numbers which ear earlier than the maybe many times artificially for debugging purposes only locally bumped version numbers. So you'd wipe that development VM. Cumbersome?
Perhaps it would be better during debugging to just bump the minor version number?
Anyhow. I weaseled myself out of this mess by using Qubes, where this is much more scripted.
1) creating / updating the local repository
2) trying it out in some TemplateBased VM anon-whonix-dev or sys-whonix-dev using https://github.com/Whonix/whonix-developer-meta-files/blob/master/debug-steps/qubes-repo-temp
3) to revert, just shut down the TemplateBased VM and go back to 1)
That of course only works for tests that don't involve booting. But also TemplateVMs whonix-gw-dev- / whonix-ws-dev can be reverted using
qvm-revert-template-changes for one change or by using