In general it's easy.
Just some hints upfront:
= There is a good MTL ad in June/04 ModelRailRoader. Compare for styles
you need. Also in an older MRR has been a nice list of freight
car trucks, their manufacturing years/usage etc.
MTL is also selling "high" and "low" flanges, and trucks are coming
with king pins, washers, etc.
Last but not least, MTL has different coupler lengths on sale!
= ATLAS also sells trucks, but I think they don't uncouple as reliable
as MTL, they might just require stronger magnets.
= Sometimes (older ConCor) the holes in the trucks need honing. No
problem with the right size of round file, just be carefull and take your
time.
= Similar w/Bachmann and ModelPower (screw instead of pin).
Try out and have fun.