Upgrading to 35 lb tanks- super quick.

We sprang for aluminum 40 pounders, which weighed about the same full as a steel 30 pounder.
Longer center rod, hard to say about the lp lines, pretty sure I changed the low pressure post regulator line and raised the regulator.

Often can be done, just be aware of the extra weight which will be on the tongue..

While heavier tongue is often desirable for best towing ,IF you are one of those folks who are right at your tow vehicle cargo ratings, the extra tongue weight may put you over..

20 lb cylinders when full are somewhere around 38-39 lbs each..

30 lb cylinders when full are going to be somewhere around 58 lbs each..

20# cylinder
H----18"
D----12.5"

30# cylinder
H----24"
D----12.5"

40# cylinder
H----29"
D----12.5"

Yes direct swap if the hardware, hoses and cover can handle the extra height.
Go right to 40 pound if you like.

The propane fitting and pressure is the same.

Make sure fiberglass cover is tall enough and your pigtails are routed so they don't rub on it. Mine rubbed a hole in one of the lines and began to leak.

First, you are talking about 20 lb (5 gallon) and 30 lb (7 gallon) cylinders. Most of the things you need to make it happen, in addition to the 30 lb. cylinders, themselves:
Make sure there is no clearance problem that hinders installation of the taller cylinders.
Probably longer lines leading from cylinder to regulator. You don't want these lines to be pulled tight.
Longer all-thread center rod to hold them in place while you're traveling.
A fiberglass cover for the taller cylinders.
Otherwise, it is a very simple changeover.

20lb to 30lb maybe.