Another consideration on this crankshaft bearing replacment question is at what point are the crankshaft journals ground and polished to another underside bearinfg size.
My thought is leave the journals alone unless the bearings are being eaten up and throwing lots of metal into the oil.
It is easy to replace the main bearings and rod bearing on this engine while the engine is bolted up to the transmission and front end.
Bearings are cheap compared to a rebuildable crankshaft, which are becoming more difficult to locate.
So if the oil pressure < 15 psig at 1500 rpm at normal operating temperature, then change out the bearings. Grind journals only if there is noticable bearing metal material mixed in the oil. This will help preserve the N tractor for generations to come.