I have a over the range microwave/vent hood in a rental I own. After
eight years the microwave failed (an electrical burning smell whenever
it's running). It's a Kenmore.

So I bought a replacement (Kenmore) and went there last night to install
it. The mounting hardware has changed so I had to put the new wall
bracket on. They moved the top mounting holes (that secure it to the
cabinet above) forward and out. Now the mounting holes have to go
through the front molding of the cabinets (which I think is only glued
or nailed on) so I reinforced it with L brackets. I also put in flat
plate brackets on top and screwed them to the bottom of the cabinet then
put the mounting bolts through the plates. One of the mounting bolts is
now so far to the right that I had to trim the outsider right molding a
tad so the bolt could go through the hole.

I don't know why they felt compelled to move the mounting holes to such
bad locations. Usually those top cabinets hiding the vent pipe are
pretty shallow.

Unrelated to the installation of the new unit, I realized that the
contractor that remodeled the kitchen and put in the original unit had
not rotated the squirrel cage blower assembly to the position for top
venting (it comes set up for rear venting). So while the vent pipe
through the roof was connected to the top of the microwave/vent hood
unit, the blower was blowing directly against the sheet metal of the
back of the unit for eight years, essentially doing nothing. Since I
didn't live there I never noticed it and no tenant ever complained about
lack of venting.