Change palette used in a tileset

Hi. This is my first post in this forum, so if I do something wrong please tell me and I'll try to solve it. I followed a video tutorial to learn how to insert new tiles in my hackrom without altering the palettes already used in the game. I followed the video and by doing so I was able to insert a new tree in my hackrom, replacing the existing one. So I decided to go on and try add a snowy tree. I choosed to add it in the tileset38 (i hack FR and that's the tileset used in icefall cave, in four island) because the palette I needed was used in that set. Again I was able to insert the tree, but free space in that tileset is very low and I can't add new snowy things. How can I deal with it?
I tried to add new things into another tileset with much more free space, but I don't know how to change the palette used in that tileset into the palette used in icefall cave (in A-Map seems that all the palettes are used in every tileset, I don't know if that is correct or not).
So I tried to replace a whole tileset by importing the tileset38 and the palette too, so I could save the tiles used in icefall cave (by maintaining tileset38 in the rom) and add more things (by overwriting a tileset whit tileset38 and then add there all the new things), but again the palette don't match and weird things happen.
Is there any solution to my problem?

Thank you for your help and the time you spent reading this (excuse my English)