Moving to threads won't *kill* your google rankings... Threads has redirectors for your ubb.c links, so any incoming hits will redirect to their proper location within threads...
Also there are mods (more specifically, my sitemap addon) which push *all* of your topic links into yahoo and google so their discovery engine is populated with your new install almost immediately (though you'll have to wait to be crawled, it'll help speed it up the best way possible).
