Later tractors had greaseable bearings, much better.
can the oil type lube system be converted to grease type in the field?
I think so, wouldn't require any machining I'm pretty sure. The extent to which it would have to be disassembled depends on whether the oiling system used pressure hoses or cups and tube. If the latter, the final drive and steering clutch housing assembly would need to be removed to enable installing a grease pressure flexible tube from the top of the transmission case down to the throw-out bearing assembly. Might be able to do that without removing the inner steering clutch hub.