How hard is it to replace a hollow-core door with a solid one?
I have hollow core doors on my pantry and I have full height wire shelves on the inside, each one chock full of canned goods. No problem. Just be sure to use hollow door anchors to mount the shelves though, not just screws and not just the cheap little plastic inserts.
This type that expands behind the door skin:

As to changing them to solid, the thing that you may find challenging is in getting the hinges in the right place. I've found that pantry doors often have less standardization than bedroom doors and setting hinges is a skill set that is not for the uninitiated (in my opinion). I'd try to not change them first.