Yes, if for some reason you want to leave RedShift, you will lose the benefits that Redshift provides and you will lose the speed. But you shouldn't have to change your program.

I'm not saying it is a good idea to leave Redshift in some distant future where you might save a few pennies. Heck, I hardly ever say it's a good idea to rearchitect a core part of your infrastructure without having a very good reason -- saving a few dollars isn't one. But from the software side, you aren't stuck with Redshift and you aren't embedding a lot of proprietary AWS specific drivers in your code base.