When overhauling bottom bracket bearings, is it possible to replace loose bearings with cartridge bearings?
Can you clarify if you mean a cup and cone housing, to change from loose bearings to bearings in a metal retaining cage? Like this:



ANSWER 1 These can totally be used instead of loose bearings.  There are fewer balls to carry the weight, and over time the cage may break down and get chewed up.  



OR do you mean cartridge bearings with ABEC ratings like this:



ANSWER 2 No, these can't be used without major re-working of the bottom bracket.  There are adapters to do this, but they won't be cheap.



OR do you mean a cartridge bottom bracket like this:



ANSWER 3 Yes this works perfectly, is dead simple, and relatively cheap.  Plus they're greased for life and never need adjusting.  When they get crunchy, replace.  I've done this on all my bikes.