Mate, Vesko now runs 17" wheels with 245/70/17 Yoko Geolander at-s tyres with 2 spares one off the rim. The roads out there are tyre killers if you don't run the right pressure and speed. The choice of at tyre for 18 or 19 inch wheels is very slim and there is virtually no chance of any of the settlements out there having any replacements in a 19" available if you rip a sidewall. In some areas the rocks on the road are quite daunting and there is a real chance you can damage your rim if you hit one with such low profile tyres. Also, your traction on sand isn't as good because the tyre doesn't bag out as much as a 17" one will. If you strike some corrugations, then the 19's won't absorb the shock as well and will shake the car apart.
Where are you based? There may be some test runs close to home that you can use to set the car up and prepare yourself. There is a lot of preparation involved in getting the car and your knowledge of off road techniques up to speed.

Totally agree with Mike, you can see the difference between the 18'' stock tyre and the 17'' Geolander:

Click image to enlarge

It's been all tears for me over the break. I had an extremely bad case of shoulder inflammation over xmas, (the pain was 5 X that of child birth) and the docs don't know why, so I couldn't go anywhere between xmas and new year. Then I rescheduled for the Australia Day week, fired up the Jeep and got to Tamworth where it blew an oil cooler seal ending that trip. I patched it and drove home 400kms at 60kph because nothing was open up there. It still isn't fixed... I've had zero 4wding when I should have had plenty. It's been great....

Hey Vesko, is it just the light or is that 18" rim showing a few bits of damage from your earlier adventures?

I was thinking just the same thing, I think it's brake dust more than damage though?

You are right, the 18'' tyre damage is after crossing of Simpson and Sturt Stony Deserts. The rim is OK, as taztastic says it is brake dust.

Mike, damn it! And I was enjoying my Laos travel looking forward to see your Fraser trip. Sorry mate.