babyg_wc Just bear in mind that if it is within 3 months of the original date you wont pay extra, however if it is less than the voucher value you don't get to keep the difference thats left.
If its beyond 3 months of your original flight date i believe you will get the difference back if it's less.
I got in this situation last week, where i booked less than ideal flight times because it was cheaper. The plan was to use whats left towards another trip I was planning to take.
No big deal really, but i would have taken the better timed flights, which were more expensive, had i know what i know now.
Hope this helps.