I'll second the bushing sentiment- It's better (and safer) to have a good bronze bushing than a cheesy low speed bearing. You will have to keep the bushings well-lubed and replace when worn to the point of excessive slop. Depending on the wheel retention method, a failing bearing can quite often result in the wheel separating from the vehicle. Not cool.
Now- with regards to replacing bushings/bearings- A crappy bearing can be driven out and replaced with a good bearing if you can find a good one with suitable dimensions. Be aware that a lot of low speed units have a built in lip or shoulder that holds it in the bore, while good ones do not. Be aware of this, you may have to fool around with the way the new bearings are held in. Replacing bushings with bearings is not so simple. Since bushings tend to be thin (like 5/8" ID, 7/8" OD), you will not likely find a bearing that suits your needs. Basically, you'll want a wheel designed to take bearings if you feel the need to use bearings.
