change 1/8 note measures into 1/4 note measures

I've got myself into a bit of a pickle.....have written 8-10 measures of 1/8 notes that I need to convert into 16-20 measures of 1/4 notes, i.e. each 1/8 becomes 1/4. Source is currently 4/4, and destination is to be 4/4/ time.
What's best way to do this without entering each note from scratch?

THX.

Dick