Not exactly sure what you're asking but...
1) Changing the mode from RGB to Lab color should theoretically have no effect on the color of an image, though in practice it can have minute effects if you go back and forth between Lab and RGB several times due to accumulated conversion error.
2) When you say "soft light" that is a layer blend mode.
If you make a copy layer of your image, then change the blend mode for that new layer to "soft light" you will at the the same time increase both color and saturation on that layer. You can then dial opacity down to moderate the effect. Some folks like to place a color layer in soft light blend mode over a B&W version of the image, then dial down opacity to get a sort of edgy look to the file. However, this result will appear identical whether you are working Lab or RGB...
FYI for our Moab participants, this is a subject we will be covering in detail during the color management session
Cheers,