This is a pretty good idea, really. It would prevent a determined 2/3 majority from changing the Bill of Rights by simply changing the threshold by which it can be amended. In its current form, however, I think it suffers from some clarity and flow issues that I will attempt to clean up. I suggest that the author adopt this version of the proposal:
Article 9. Amendments
1. The Regional Assembly may amend the Bill of Rights by a three-quarters vote.
2. The Regional Assembly may amend this clause, the preceding clause, and Article 1 of this Constitution by a three-quarters vote.
3. The Regional Assembly may amend any other part of this Constitution by a two-thirds vote.
This version makes things a bit more clear, I think. You'll note that I've left Article 7, Clause 14 out of this version, because I think that if 2/3 of the RA no longer wanted the constitution to be supreme law, that's enough. We shouldn't be picking and choosing parts of the Constitution to be harder to amend, just because we think they're more important. This fully closes the loophole of being able to amend the BOR with 2/3 vote, and we should probably stop there.