Trans swap?

no.
the final drives are the same, an 18 just lets you split the basement. look on eaton's website and compare the 18 and 13 in terms of drive ratios. that's the difference. if you feel you are reaching too far (hauling heavy and need to have shifts tighter on rpm) between gears on a 13 when starting out slow, go for the 18. the 9 series trans are the newest according to the eaton rep i spoke with at length when ordering a new trans. rtlo is their "low inertia" style trans...read "more money bc it's the newest thing" i ended up going with a rebuilt 7 series. RTLO16713A. works for me...saved like 5g's from a 18 speed 9 series. still wish i would have gotten the 18 though. better to have and not need something than need and not have.
Look at eaton's website.
If you want taller/higher topend, compare your final drive with those available..and or go with the 18 and put a lower ratio into your rears.

 

i saw your other post regarding the upgrade, just forgot to respond :/
friend of mine just swapped his 15 for a 13 like mine. picked up .6 mpg because of the difference in gearing...he had a 15 deep reduction. could drag anything anywhere...except on the highway. final drive was geared to close to 1:1

 

Depending on what model your 15 is you may not see much of a gain in top end, they did sell an overdrive 15spd.