From this post, I got the idea to remove and replace the wheels altogether using a hacksaw.
Here’s what it looks like right now. My problem now is that I can’t remove the leftover metal shaft from the caster wheel.
I’ve tried using some lubricant and hammering it with a screwdriver but it didn’t budge.
I don’t have an answer, but I can smell the grease from here.
I would expect that the wheel shaft has a flange or maybe some knurling on it to prevent it from coming loose. You will probably need to drill it out. I wish you the best of luck on your journey, you will learn a lot about how things are made. I also expect that this will end with you buying a new castor or a new suitcase.
Once you take it apart, you’re unlikely to be able to buy new wheels. Better to replace the whole unit - I did this just last week with an office chair.
Pull the entire black part out of the gray part but don’t throw it away yet. Bring the old castors to the hardware store to check the size and shape when you buy new castors. In Australia, new ones are sold like this:
https://www.bunnings.com.au/move-it-50mm-black-hooded-office-chair-castor-2-pack_p3940114
You should be able to find something similar where you are.
Can I recommend rollerblade wheels? AKA inline skate wheels.
You can upgrade while repairing.
I assume the metal axis is probably molten firmly into the plastics with some kind of knurling. So probably you are going to break the plastics if you are trying to push it through. As suggested by others, a solution may be to try to drill it out. The other option I see, is to remove the whole black part and replace it by new rollers which have standardized shafts are when dedicated for furniture.