While there are a couple programs for converting .psd files from CMKY to RGB, only one freeware program that I know of correctly handles layers: XnView.
Before you go straight to downloading thinking your problems are solved, there’s a catch. XnView can import hundreds of image formats and convert them to 70 or so formats, picking any output format won’t necessarily work. It turns out, converting from a layered psd in CMYK to a layered psd in RBG results in a psd file that Gimp can’t handle (much less anything else I know of). However, what you can do is output all layers to a series of .png file. To do this, in the program, go to the output tab and, under the area titled “Multipage”, check both boxes: “Convert multipage file to multipage file (when possible)” and “Convert all pages from multipage file”. I haven’t tried what would happen if only one was checked, but basically with this setup, every layer will be output as its own separate file.
Why not another format?
First, many of the formats aren’t common, and its possible that multiple conversions may result in a loss of quality.
Second, the common formats, such as bmp (bitmap), jpg (JPEG), and gif, don’t suppose transparency (bmp might, but if it does, it’s always fussy for my programs).
Are there other options to XnView?
… Sort of. There is a plug-in for Gimp called Separate+, but it isn’t updated very often (so it may be buggy), and, more importantly, it doesn’t support multiple layers in a psd file.