I'm hoping my stupidity may help someone else. Fortunately, no damage done to flesh, just to pride and metal...
First mistake: Starting to replace the blades when I was tired.
Second mistake: not picking the heavy planer up off the floor to work on it. [Ended up having to do so anyway later.]
Third mistake: using my DW 18v impact driver to replace the bolts that hold the blade to the cutter head. [See item 1]
Fourth Mistake: Snapping off one bolt [an end one, of course] with the impact driver.
Fifth Mistake: Thinking [see item 1] "it's only one bolt, the rest will hold"
Sixth Mistake: Turning the planer on.
Seventh Mistake: telling anyone about items 1-6.
So, the centripetal force of the blade and holder spinning at 10,000 rpm tore the cover partly off, and broke the blade in two. Took all of about 1 sec. Maybe 2. Spent the next 90 minutes or so taking off the blade and drilling out the old half bolt and rethreading [metric] the hole.
Fortunately today I found a fellow kind enough to sell me one blade - I'd hate to have to trash the other perfectly ok blade, and an internet source for the blade cover...which will get here in a week or two.
Just sharing to hopefully prevent someone else from doing the same thing.