Rick,
The Major was never designed to change on the move. You should select the gear you need and start off in that gear. If you have a "Live" gearbox it is virtually impossible to change on the move. You are moving massive gears and the loading and pressure on the shafts does not allow slick changes and the braking effect of the transmission can leave you parked by the side of the road if you try a double declutch.
Having said that, it can be done if you are careful to match speed and engine revs. I tend to use the high/low lever to change up and down when loaded, on the road.
However you cannot really change up through the gears as the jumps between them (4th to 6th) is rather large and any other way would mean shifting both levers.
In hilly country I would not advise trying to change on the move as you can soon find out how good your brakes are
Dextas are different and it is quite possible to shift up and down the box (if you can remember where the gears are) using the double declutch method. If you are really really good you can even do it without using the clutch at all

but here you have a lighter gearbox and a more free running tractor than the Major.
A little story about shifting on the move. A few years ago back in the 1980's Massey brought in an electronic gearshift along with electronic controlled hydraulics. We got used to stories, and I got called out to a number of tractors, that kept lifting the plough out of the ground when crossing the field near power lines.
The best one was the man who thought they were so good that you could instruct the lift what to do on the CB radio. He demonstrated this to his boss by standing beside the tractor and saying "lift the plough" into his CB mike, then pressing transmit and the plough came off the ground!
However the tractor going down the road with a load of grain, in top gear, that changed instantly down to bottom gear as it passed an electrical sub station was rather dramatic!