The histories of Iceland dont mention which 2 companies of the 21st went in July `41 & were redesignated as the 824th. To find that out you`d have to get the Regiment`s records from the National Archives in Collage Park, MD. One history says only 2 companies went, another says the 1st Battalion minus 2 companies went. Since there is no record or mention of any other part of the 21st after the redesignation, it was most likely there was only the 2 companies. Also there is no further mention of the 824th, they have no campaign credits & dont appear to have ever left Iceland. Again, you`d have to get the unit records to see if any of the men were transfered to other units.
An explaination of your gdad going in Jan `42 probly has more to do with the historical timing. It appears he was drafted in April `41 which was only 3 months before the units of the 21st left for Iceland. Even if he was in those units at the time, the Selective service Act of 1940 prohibited draftees from serving outside the US and US teritories in the western hemisphere. In 1941 Iceland was a foreign neutral, and it took a lot of diplomatic wrangling to even station any US troops there. The units that went to Iceland had to be combed thru & men transfered so only regular army men were sent & not draftees. After war was declared in Dec `41, the hemishere restrictions were lifted on draftees.
I have to go for now, more later. meanwhile here`s some more reading for you:
http://www.ibiblio.org/hyperwar/AAF/VII/AAF-VII-9.html