If your publishing a public package, your better off just publishing it on NPM, as that is what most developers are used to.
I use GitHub Packages at work and the only advantage is that is effective free for hosting internal packages, as we are already paying for GitHub anyway. If it wasn’t for the zero price we wouldn’t be using it.
If you really want to force all your users to migrate to GitHub packages, and have to set up npm to work with it you could mark you old version npm deprecated
and use that to point people to the new version.
https://docs.npmjs.com/cli/v6/commands/npm-deprecate
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