Your JK (2007 and newer are JK, not TJ) is new enough that I haven't done speakers in one but if it's like the earlier TJ, it's a piece of cake to swap the front speakers. Look to see if there is a screw on the side next to where the front speakers are, the door needs to be open to see if it is present. If that screw is there like the TJ uses, remove it and then the grill that covers the speaker can be pulled off. The TJ's grill is held in place by snap-in steel clips once that side screw is gone, hopefully the JK is similar.
The Polks will sound good, they are a more sensitive speaker than usual so they don't need as much power to drive them to make them sound good. And a big welcome to JF.
And while the factory installs 5.25" round speakers in the rear sound bar, 6.25" speakers drop into the same opening without any modifications needed. That sounds wrong I know but it's true. At least that is true for older TJs, no telling how the JK's rear speakers are installed but it wouldn't surprise me if that wasn't true for the JK too.