When I was cleaning my mailbox I found some great information about the differences between the RG400 and the RG500, so here it is.
1. Carb slide needle and main jetting: 400 = #135 , 500 = #120.
2. Clutch: the 400 has some friction disks less (1 or 2, I’m not
sure). See if your clutch will work OK so you may leave it as it is.
3. External sprockets and chain, 400 = 15 front /41 rear, 525 chain,
500 = 16/40, 530.
4. 400 pipes have only thiner stingers, all the bodies are the same.
5. Cylinder, piston: 400= 50mm 500=56mm
400 has 5 intake ports, 500 has 7 intake ports but smaller (but 400′s
port in total I think give better flow – i.e. less divided area).
External lower sleeve diameter 400=60mm , 500=66mm
you need to rebore the upper crankcase.
But I’m not sure if you can rebore your 400 cylinder sleeves to reach
56mm so you won’t need any crankcase machining (and won’t need to buy
a 500 cylinder set). Remember the 400′s cyls may give a better intake
6. Heads diameter accordingly are of: 400=50mm , 500=56mm.
7. Conrods are the same, only its lower bearing consists of some more
(1 or 2, I’m not sure) needles so they are closer each other. You may
not bother splitting your cases and keep your 400′s bearings though if
you go arround the stock 500 horse power (no much more).
8. Some small difference in the front brake rotors. 400′s are no
9. The rotary valves have different base, the 400′s is thinner and
lighter so keep your 400.