Is it possible to change base tileset?

Unfortunately I require some cliff tiles that are not included in the base tileset I chose a LONG way back in the development process. I was wondering whether anyone knows of a way in which I can either:

1. comletely change the base tileset on my map without losing important stuff like the layout of terrain, trigger data etc.

2. There is in fact a way to change the "restricted" cliff tiles.

i used the modify tileset thing but it never fixed cliff tile so i dunno bout cliffs i wanna no cliffs to someone help us!!!

Advanced -> Modify Tileset -> Change Basic Tileset would cause such losses? :\

The general layout would remain the same, but if the new tileset has more/less overall tiles some things may become odd looking, and need a quick touchup. Trigger data however, will not be messed with.