Updating a texture with the same name has no influence with the Cache , you can try with turned off cache but this will not make any difference. the cache will update as soon as you render.  you just can't  rework a texture while it is loaded in poser and expect that it will change in the viewport as soon  as you overwrite the Image. Same happens in blender btw. no constant refresh of loaded textures if you think you can update rework repaint textures in real time over a 3rd party paint app this will just not happen