I believe it is Microsoft that is doing more changing than Yahoo. Yahoo supplies Microsoft its results, so if Yahoo were changing, then Microsoft would be reflecting the same changes. The new Microsoft search engine should be up and running by the end of this month per online rumors and press releases.
MSN search is comming and is quiet possibly part way here
Any fluctuations you are seeing at MSN do not have anything to do with the new search engine they are working on. The release scheduled for tomorrow is simply a beta release. Yahoo is still powering MSN's search.
i'm seeing one of my sites that incurred the yahoo chop, but was doing well in Msn beta now showing on MSN results, maybe they are migrating results across slowly or maybe yahoo likes my site again, but it hasn't appeared on Y yet, hope its the latter.