UCR and GCR in Photoshop

I work in printing and sometimes I run into images that have too high of an ink density in them. In all cases I have seen it is in the black or dark areas of the images. I usually try to keep the ink percentages under 300%. These areas usually just have more Cyan, Magenta and or Yellow than they need. I believe what I have to do is referred to as UCR or GCR. I'm not exactly sure which. If it is a small amount over 300%, I can usually make a 'Selective Color' adjustment and reduce the CMY from the blacks and there is no difference in the image.
My problem is when the numbers are like 340-380% and there isn't just one small area in the image. The spots are all over. What is the best way to fix something like this with out affecting the color of the image or affecting it as little as possible. I've been told that converting the image to RGB and then back to CMYK works, but in some instances there is a notable color shift or at least a visible color shift. I've also seen that converting the color profile to a SWOP profile will adjust he photo for this, but is this a safe way to do it.
Can anyone help or point me in a direction to get help? Thanks.

Some sheet-fed presses can handle 340% Total Ink on good paper (but don't assume yours can). On most presses, if the area of offensive density is small (say: a quarter square inch) there's no danger.

Converting to RGB shouldn't be necessary if you work in... RGB to start with, and convert to the press' CMYK profile your flattened (final) file.
If you don't have a custom profile, use Adobe's SWAP varieties (sheet-fed or web).

If the file was CMYK when you got it, honor its profile (if it has one) and than convert to the custom profile.
If the color change is too drastic, undo the conversion and adjust the CMY under the Black with a masked curve layer (make the mask from the Black channel, but take the midtones down a bit).

You can try to repair the color damage by placing the original image as a new layer over the converted image and changing its blending mode to hue. You may also need to add a second layer of the original with the blending mode set to color. For two layers, you will probably have to adjust the layer opacities.

You will have less color shift if you go to Lab and back.

If you need to do this a lot, you might want to try Intellihance Pro from Extensis which will reduce the DMAX of CMYK images without affecting ANY of the pixesl which do not exceed the total ink limit you enter into the filter dialog. Start with the original image then go into fine-tune mode and look for the total ink setting.

Costs about $80USD. Can't vouch for version compatibility... last time I used it was with Photoshop 6.