EDIT 1: PayPal and LinkedIn indicate that the migration app's developer is in China. [1] That initially gave me pause about using the app --- who knows where my information might end up? --- but then I'm guessing that Chinese hackers seeking to steal confidential information aren't likely to be so open about being in China, nor to charge $30 for the app. (Of course, that too could all be part of the evil hackers' cover story, right?)
EDIT 2: I just used the app to migrate 6,500-plus notes from Evernote to OneNote as a test. It seemed to work flawlessly. (My suggestion is to use Evernote's export facility on each EN notebook in turn to create a separate export file; then set up OneNote sections corresponding to each EN Notebook; then use the OneNote Bulk app to import each EN notebook's export file into the corresponding ON section.) I'm still pondering whether to make a permanent switch.