If you're easily bored, skip down to the first pic.
I started with a cheap Princess Auto 3 shelf cart. My use is to primarily keep this empty, fill it with parts and tools when doing a job, then put everything back away when I'm done. Of course things don't always work out that way, it sometimes becomes a "stuff" collector.
Anyway, I have a lockstone path from the back door of my garage (where the tools are) to the front of it (where the cars, bike etc are). So the idea is when the job is done, all the accumulated tools and old parts can be wheeled back to the back door. The wheels were too small for this to work well.
I got this idea from the HF cart mod thread. What I started with.
New and old casters
The original caster threads directly into the end of the post. 10 mm threads!
So the corner bolt needs to be a 10 mm. If I could find some 10 mm coach bolts, I'd use them for the other 3, just to standardize. Now I have one temporary bolt in the opposite corner. I need to run off to the hardware store to get some 5/16 coach bolts and nylock nuts.
I put a layer of hardboard between the casters and bottom shelf to lessen the local stresses on the shelf.
The casters will hit each other if they both "toe in" when changing directions, but can be forced past each other.
I moved the bottom shelf down about 1 1/2 inches to sit on the hardboard. The bottom shelf bolt would require drilling through the threaded insert in the bottom of the leg, so I just used them to fill the exposed hole.
And here's what I end up with. 5 inches taller! But much more bottom heavy, which will add to stability. The center of the tire can be a lot closer to the center of the cart when pushing it sideways due to the larger casters, but with the extra weight down low, it does not seem tippy. And it rolls down the lockstones and over the door threshold much better now.