You can use some parts on 511 like suspension arms, camber mounts, damper towers, all plastic parts etc.
Some parts would fit with minor mods, like making new holes etc.
Let say you want to update your 511 to 503 specs. In short, you want the longer chassis with new motor position and new 12 degree kickup.
You would have to buy (probably not 100% complete list):
alu chassis plate, rear (front) suspension mounts, center bulkhead (right), motor mount, new topdecks, new front bulkheads (upper and lower) with bulkhead spacers, new drivebelts, new front suspension mount.
Center bulkhead mount is shorter. So the 511 bulkhead does´nt fit the 503 chassis unless you make new holes in chassis. Could be done of course.
Front upper bulkhead is modified to fit new kickup and is now 12 degrees, 511 had 8 degrees. Not a new 502X mistake...
You can use 503 upper bulkhead on 511 and vice versa. Basic dimensions are same, but angle on damper tower would be wrong of course.
503 damper towers fit 511 and vice versa. Rear tower is basically same, with outer holes removed and inner added. Wing mounts are lowered ~10 mm and moved out ~3.5 mm.
Front tower sit ~1.5mm higher. Outer holes removed.
Front suspension mount to change kickup. Also front lower bulkheads are modified for new kickup of course. (Want the suspension mount to be in the correct angle)
You can then add gear diffs, BB dampers etc to the list and you will realise that buying a 503 is a better option.