No, K66 is not a drop in replacement for K60. K66 has one I2S, K60 has two, K66 has less FTM and ADC. K66 has two USB ports, K60 has one. The MCG is quite different. So the added ram seems to come with some cost. The extra speed has a drawback too. Running 180MHz the K66 can not flash, you have to temporarily switch to a lower speed in order to flash. So depending on your application you may have to rework your PCB and software. But it may be simple also. Check every pin, if you have a 100% match just recompile with the proper MK66F18.h and startup files.
First , you may would like to have a look at this post:Can we use MK66FN2M0VLQ18 as a drop-in replacement for MK60FN1M0VLQ12 ?
Actually one important thing to notice is that K66 has 2 USB modules (USB HS and FS), it is important that for USB HS module, an internal PLL needs to be activated (This is exclusive for K66 and it does not appear on K60 nor K64).