How to update fork of old (pre-eclipse) repository?

Hi, I would like to update my github fork of the old repository, so that I can get started with a pull request about some DL4J-UI issues.
My last PR was #7336 on Mar 25 2019. Since then, the deeplearing4j repo was moved to eclipse, on Jun 6 2019.
So in my fork repo, I have renamed remote upstream to old_upstream, and added the new URL as upstream:

$ git remote -v
old_upstream (fetch)
old_upstream (push)
origin (fetch)
origin (push)
upstream (fetch)
upstream (push)

Then, I fetched upstream and merged upstream/_old/master.

$ git merge upstream/_old/master
$ git status
 On branch master
 Your branch and 'origin/master' have diverged,
 and have 285 and 3 different commits each, respectively.
   (use "git pull" to merge the remote branch into yours)

nothing to commit, working tree clean

Then, I tried to merge upstream/master, but this didn’t work:

$ git merge upstream/master
fatal: refusing to merge unrelated histories

How can I get to upstream/master from this state?
Or do I have to delete my fork and fork the new eclipse repo?

Thanks in advance.

When DL4J was donated to Eclipse, the history of the repo was lost, so you can’t do it that way.

So, you will have to start over from a new fork. You can however extract your own changes in a patch format, and then apply them again after cloning the new fork.

I have not made any changes since my last PR. I will start working when I get to an up-to-date code.
The problem is that I can’t make a second fork (this is a limitation of my github account).

Is there any way to transform my fork to a form that can be merged to eclipse/deeplearning4j?

I found what I was looking for: hard reset to upstream/master:

$ git reset --hard upstream/master

Thanks for the help, @treo