I need to change a simple piece from 4/8 to 4/4. Do I only need to double the values for each note? (Ex. quarter to half note & eighth to quarter?)
If you wanted all your notes to have the same value in the new meter, you would double them to the denominator of the time signature being cut in half. For example if you had a measure like this:
It would become this:
Notice how all the eigth notes became quarter notes and all the sixteenth notes become eigth notes. Also if you want the tempo of your piece to stay the same, you also need to double it.
Short answer: yes. Simple double all rhythmic values and change your unit tempo to match your current unit division. For example, if before, the music was at 8th note = 120bpm, after the unit division change, a quarter note = 120bpm.