Certain strategies change, but not the core. The core fundamental of the character remains. You'll never make Zard a rushdown character, or you'll never make Ryu a zoning character. some characters have a flexible core that allows them to engage in a variety of playstyles, but that only makes them an all-rounder.
I know there are some characters that are not really all-rounders, but are still pretty flexible in use. Zard is mostly a turtle because he is better at punishing attempts to approach him than actually approaching his opponent. However, he can still approach and sometimes need to. Having a flame thrower, quick skid animation, fast and mostly safe jab combo, etc... that's not what he excels at though.