It's not likely, but not impossible, that your legacy code will work without change. Most migrations from S12 to S12X require the usual review of memory map, peripheral features etc.
There should be no real need to consider the XGATE in your loader.
One feature you can probably take advantage of (and I'm 90% certain it is not supported in the S12) is the ability to set the vector base address. This lets you more easily utilise interrupts in your loader if that is something that would be useful to you.
Yes, VBR is certainly an advantage, but also probably a problem for Anders. S12X doesn't allow any resource remapping, there are no more INITRM, INITRG and INITEE registers. Old Freescale S12 bootloaders were moving all bootloader code from flash to RAM and remapping RAM to the vector area. Serious code modifications are required to port such S12 bootloader to S12X.