Comparing the net benefits:
- Net benefit per batch (gallon-size containers): \( \boxed{2438} \)
- Net benefit per batch (individual portions): \( \boxed{3162} \)
Since the net benefit from selling individual portions (\( 3162 \)) is greater than that from selling gallon-size containers (\( 2438 \)), Naturalmaid should convert the plain yogurt into individual-sized portions of fruited yogurt.
Thus, the final recommendation is:
\[
\text{Naturalmaid should convert the plain yogurt into individual-sized portions of fruited yogurt.}
\]