CASSANDRA-19615; Merge pre-existing schema with the system defined on…

[krummas]

…e during upgrade

Thanks for sending a pull request! Here are some tips if you're new here:

• Ensure you have added or run the appropriate tests for your PR.
• Be sure to keep the PR description updated to reflect all changes.
• Write your PR title to summarize what this PR proposes.
• If possible, provide a concise example to reproduce the issue for a faster review.
• Read our contributor guidelines
• If you're making a documentation change, see our guide to documentation contribution

Commit messages should follow the following format:

<One sentence description, usually Jira title or CHANGES.txt summary>

<Optional lengthier description (context on patch)>

patch by <Authors>; reviewed by <Reviewers> for CASSANDRA-#####

Co-authored-by: Name1 <email1>
Co-authored-by: Name2 <email2>

The Cassandra Jira

[beobal]

nit: would it make sense to name these newKs/oldKs or similar? Not a big deal really as there's only the one usage of the method for now.

[beobal]

1 tiny nit/question.