I just spent the better part of the day on a design for business cards, only to find out now that I was working with the wrong file format. Basically, I end up with my PS document with layers (RGB) and now I need it in CMYK at 300dpi. Can I just save as is, then open a new document at 300dpi, in cmyk, then pull my project onto the new (correct) format document, and resave? It can't be as easy as that can it? Probably not.
Open your original file and click on the Image menu. Choose Mode, and select CMYK. It will ask you if you want to flatten the image, keep in layers, or cancel. If you want to keep your layers, choose that one. Then save it (as a new file if you want to keep the RGB version as well).
If you have created the file in less than 300dpi resolution, you cannot upsize it to 300dpi without a lot of pixelization occuring. In that case, you will need to redo the artwork.
To check your image size, choose the Image menu again, and select Image Size... This window will show you your resolution.
You can increase your dpi IF you reduce the actual image size by an equal amount. For example, if you have a 72dpi image that you want to upres to 300dpi, that is approximately a 4x increase. Therefore, you have to reduce the pixels across and down by 4x. Meaning, if the image is 400 pixels wide by 400 pixels high, you would have to make it 100x100 to compensate for the corresponding increase in dpi.