Hi
Using external SDRAM is essentially the same as using the internal SRAM but you need to initialise it differently. All you need to do for SRAM is to change RAMBAR, but SDRAM requires more initialisation (dependent on your hardware and implementation).
There may also be some differences in bus width as the SRAM is on the internal 32 bit bus and your SDRAM may be on a 16-bit or DDR bus, but again there is no difference from the programmer's perspective once you have configured the SDRAM controller.
Cheers,
Paul.