It's a fiddle to get the bearings correctly tightened, as you seem to need three hands.
Otherwise no problem as long as you don't lose any ball bearings.
Pack the bearing with grease as that stops water getting in even if the bearings don't need it all for lubrication. Any excess will just get squeezed out.
Check the tightness when the wheel is on the bike and the QR is done up as this will effectively tighten the bearing.
Make sure the locknuts are TIGHT on the cones, otherwise the cone might rotate and seize the bearing.
I've just remembered why I like cartridge bearings now.