i just converted my US driving license to SL driving license using the one day service.
All i needed was
1/ My US license which had to be current ( cannot be expired)
2/my passport with the valid US visa
3/original birth certicifate
4/SL ID card
5/medical certificate: there are 2 versions for this. i used the old one and had no problem. the new one looks more professional and you need a photograph too.
my advise is take copies of each when you go. Get in line by 5:30 in the morning. they only give out 200 numbers for day service. it costs Rs.2500.(they say 2200 but inside they charge for international conversion fee of 300). you can get the card by 2pm the same day.
outside is a little messy but inside everything is pretty straight forward. standing in line is not fun. they take 50 people at once.
good luck!!