The H21a hoppers underwent extensive modifications in the 1930's & '40's, including hatches & interiors.
Different interior bracing configs (tube, angle and gusset types) were experimented with.
These photos of a bashed
Bowser GLa type hopper show an example.
Both Bowser and Tichy made bracing kit parts for the GLa and H21a types, and some of the Bowser kits
If the cars you're talking about were in kit form, then they should be removable. Otherwise, they're likely cast-in, and will need surgery to remove.
BTW, if the H21a was ever used to haul ore, I've never heard of it. But for modeling purposes... heck, go for it.
The photos show a bashed Bowser GLa kit before & after paint (inadvertently swapped top & bottom). It's one of a string of several empties for staging. The braces are Tichy parts. The 'before' picture has a promo photo of a decal set in the background.