To the OP - I'll bet a roller bearing with good grease will outperform and outlast any hub assembly.
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Several reasons people consider the change from roller bearings to unit bearings. A few reasons are:
Wheels. Unit bearing hubs open up a large variety of factory wheels that will not fit the roller bearing hubs without adapters or wheel/rotor mods.
Brakes. The braking on the roller bearing assemblies has always been considered marginal by most owners. The standard brakes for the unit bearing hubs have noticeably better braking. Brake service is MUCH faster and easier and rotors are cheaper for unit bearing units (unless you are content to just toss on a set of pads and not address the rotor surface - I'm not).
As to the service life issue of the unit bearings - while I hear many people complain about short unit bearing life, personally I have never had that issue. The several trucks I have owned with unit bearings all made it 150K+ on the factory hubs, and by using mid grade or premium replacements I've never had a failure of a replacement hub - I have 75K+ on the replacement hubs on my 99 Dakota. One thing that may factor in is that I run pretty much stock size wheels and tires - I can see where larger aftermarket rims or more offset may wear out unit bearing hubs faster. So while roller bearings could be stronger and last longer, in my world this is a non issue.
Back to the O/P - the consensus is that to swap to the unit bearing rotors, since the parts books show different part numbers, you need to swap all the way back to the frame rails - brakes, spindles, and control arms. I have never actually heard of anybody with greasy hands actually trying to just swap the spindles and brakes - the posters seem to just grab everything and swap it all. The spindles are different P/N's due to different caliper and bearing mounting areas. The control arms are different P/N's, due to the difference in ball joint mount styles. I've been keeping an eye out for a suitable donor truck at my local yards and I plan on trying to just swap spindles (and brakes, of course) and see if that can be done.