And don't **** about buying bearing drifts.
If you only have to worry about the outer or inner race, take a slitting disc on an angle grinder, and cut a slit radially straight through the side of one of your old bearings. This should allow you to lose the balls, and recover the two races, which, now they have a bit cut out of 'em, can be used to drift in your new bearing but won't stick in the hub / on the axle.
If you have both hub and axle to worry about (which you probably will for the second bearing), leave an old bearing intact, but flap disc the outer race to reduce the diameter by 1/10mm or so, and do something similar to the inner race (die grinder, drill with a dowel and a bit of sandpaper on it, rat tail file, etc). You can now use your newly "clearanced" bearing to drift in the new one, again, without worrying about it jamming in place. You do need to be a bit more careful twatting it with the hammer, you need to hit both races flat.
When putting your new bearings in, put them in place and tappity tappity tap around the outside until they start to go in straight. Then drift em in. Saves knackering the hub.
