Yes good advice..The cost difference in tires between a 15" and 18" rim will mostly cover the costs of new rims. Then you'd also have the 18" rims to sell, so you'd possibly even be in money.
In the KO2 there is nearly a 50.00 difference in price per tire when going from an 18 to 15 wheel. That's 250 bucks counting the spare PLUS you can sell the 18's to a JK owner and be ahead.