Key changes in orchestral part

Originally D major, at a certain spot changing to G major, so some of the sharps disappear. Sometimes (or is it often) when such a change is at hand, the sharps that disappear are put there as "naturals" (don't know the english term for this symbol, it is neither sharp nor flat but.....) for courtesy. Not in N3. Why?
Raymond
[sorry, edited the original key, wrong typing]
Raymond
[sorry, edited the original key, wrong typing]