| Joyd |
They've intentionally not added creatures to SM/SNA without bound because the last thing that those spells need is to grow in flexibility and power as the game grows. A DM can allow specific swap-outs, but a DM should know what he's doing before doing that, and should try to swap out monsters for similar monsters. (Note that matching CR alone is not a good rubric for making swaps; there are some monster abilities that are extraordinarily good in the hands of PCs.)