There are several possible options you might consider and try:
1/ In A660 there is bmap_blit, but I don't think that is available in A405.
2/ Give the entity you want to change an action script where you use:
my.skin = 5;
ent_morphskin(my,"newskin.tga");
I think that will only change the currently selected skin.
3/ Make the skin file a multiple frame file: "newskins+5.tga".
Here you will have to copy the textures you use for skin 1-4 into the first
4 positions of the new texture, and the new skin 5 texture into position 5.
Then change the complete skin of the entity in an action code:
my.skin = 0;
ent_morphskin("newskins+5.tga");
That will swap all skins of the entity.
These are just a few ideas,
cheers,
tindust