Andy, the reason you put coilovers on is that you assume they've done their homework and put a spring/shock package together that matches. Among other things, Flex gives you flexibility...in adjusting your compression/rebound. Not separately unfortunately, but you do get adjustments, plus pillowball upper mounts. In short, no, changing the springs in the basic to a flex won't make it into a flex. Will it serve any good? Maybe, but then you should have ordered the coilovers with the spring rates you wanted. If you don't know what kind of rates you want anyway, chances are you won't notice much difference.
I say just leave it.
