Remote rejected (shallow update not allowed) after changing Git remote URL

As it seems you have used git clone --depth <number> to clone your local version. This results in a shallow clone. One limitation of such a clone is that you can't push from it into a new repository.

You now have two options:

1. if you don't care about your missing history, take a look at this question
2. if you want to keep your full history, then continue reading:

So, you want to keep your history, eh? This means that you have to unshallow your repository. To do so you will need to add your old remote again.

git remote add old <path-to-old-remote>

After that we use git fetch to fetch the remaining history from the old remote (as suggested in this answer).

git fetch --unshallow old

And now you should be able to push into your new remote repository.

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Note: After unshallowing your clone you can obviously remove the old remote again.

In case your repo is origin, and the original repo is upstream:

git fetch --unshallow upstream

Another option if you want to keep the repo as is with the new commits you have added since the shallow, initial commit is this: Amend this commit with an interactive rebase.

• Start an interactive rebase including the first (root) commit with

  git rebase --interactive --root
  

• Change the pick of the initial commit(s) to edit and save & close the file.

  If you've cloned the repo with greater depth than 1, you may need to do the same for all of those commits. Or, alternatively, execute fixup for all of these during the interactive rebase.

• Convert this commit to a regular, unshallow commit with

  git commit --amend --no-edit
  

  This will also change the commit ID and add you as co-author to this initial commit.

• Don't forget to finish your rebase

  git rebase --continue