I also own a Highland Park 8" all-in-one unit. I have changed the
bearings without mishap. If you have loosened the allen screws, and
the bearing will not budge, perhaps it is corroded on.
WD-40 time. Soak it down, whack on it a bit so the WD-40 or Liquid
Wrench penetrates, let it sit for awhile, then whack on it some more
and hopefully it will loosen up.
Also, if you have enough play with your grinding wheel, perhaps you
can loosen things up by yanking the wheel towards the bearing, as a
sort of built-in slide hammer.
Dos Manos Jewelry