I suggest another option: wait to see what it's like.
Even if it's just a test, it seems a reasonable scenario that just the test will cause permanent damage. I understand trying to promote better discussion, but this system is downright unwelcoming to any newcomers. When I registered to comment I thought the downvote restrictions were odd and I was willing to try it out, but if this was in place I'd have closed the tab and never come back. Maybe subject-matter experts…
That said, Hacker News is a high enough quality forum that it could fare quite well for some time even if it completely cut off registrations. I don't think the experiment will be catastrophic by any means; though it'll undoubtedly result in at least some useful content never reaching daylight.
Of course, the fact that the HN comment sections are right now orders of magnitude higher quality than the internet at large is precisely why this change seems unnecessary. HN has long been- and still very much remains- a secluded mini-Reddit for startups and hackers. Doesn't that ethos almost demand the welcoming of newcomers?
EDIT: I wonder if everyone realizes the feeling of being excluded arbitrarily by karma count. Sure felt weirder than I anticipated while this comment was pending.