Ok I've run the question past Digicel and this is what they said:

"Prepaid vouchers give specific airtime. When the airtime expires, the remaining credits cannot be used until the number is reactivated.

You can view airtime for each top-up here: Check Digicel Airtime

You may also dial *124# to check your airtime to see when your credit will expire."

I then asked "But the credit just doesn't disappear, you just can't use it until you top-up, then the previous credit is still useable?"

"That's correct"


Hope this helps

~Moose