LM3S6965: Can I use higher speed rated parts for a current design?

I understand the Stellaris Microcontrollers are discontinued, and we are having finding the part we need. To keep us in business while we upgrade to the Tiva C series MCU could we use a 50MHz version of the part even though we designed the system for the 25MHz clock? Would we be able to use the 50MHz but with a 25MHz crystal without any further changes (since the registers are set to 25MHz speed for I2C and whatnot), or would we want to replace the clocking crystal and change the code (and capacitors) to make the system work the same for the 50MHz clock?