Frankly, unless you plan on trying to find an original SA stock to go with it, replacing the bolt with an SA won't bring the value up much if at all. The serial number of the rifle will also influence value, as most people desire a WWII era serial number.
If you want a SA bolt, I do believe that if you were to purchase a HRA Garand from the CMP there's a high chance that it will come with one. I've heard that the CMP purposefully mixes parts so people don't buy the lower grades and try to flip them for profit. It's also a great excuse to buy another Garand.