Bearings themselves are cheap and easy to replace, but if the cup and cone bearing surfaces are rusted are pitted, then it's not worth it. The cone WOULD be replaceable IF you could find the parts, but very unlikely. If the cup surface is worn or rusted, then you have to throw the whole pedal away. Replacing pedals every year is crazy, except maybe for a racer. So, I'd suggest buying new pedals and overhauling them every time they start feeling less than smooth, or buying a good pair with good bearing seals.
Oil is good, grease is better.
2007 Peter Mooney w/S&S couplers/Terry Butterfly
1993 Bridgestone MB-3/Avocet O2 Air 40W
1980 Columbus Frame with 1970 Campy parts
1954 Raleigh 3-speed/Brooks B72