Do you know something is wrong with yours already? Or are you hoping to upgrade to stronger ones?
Just wondering. I know you're planning to rotate them for more caster and just wanted to mention that you do not necessarily destroy the old ones getting them off.
If you're even remotely careful when grinding/cutting the old weld off, your originals won't even get stressed. They're lightly pressed (sometimes a little tighter) on to the ends of the tubes, then welded in place. Cutting the weld only will let you rotate your existing ones on the tubes.
Most report a little whacking with a hammer gets them started, while some few have to muscle them a bit more.
You may already have known all that, but I was making sure.
Paul