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.