Kinda MHO: They are somewhat interchangeable in that they both add dextrins. Indeed, Carapils is also known as dextrin malt. Dextrins, regardless of the form in which they enter the beer, will contribute to both body and foam. Obviously, a refined powder won't add any of the complex compounds that naturally come along with a (whole, malted, stewed, kilned) grain.
If you're doing a mash, you can just replace the Carapils with more base malt and mash a couple degrees warmer.