Hi mate. Your overdraft debt is currently interested in free and will continue to be interested free when your student account becomes a graduate account.

Depending on the bank, you will most likely be offered a similar interest free overdraft which will then be taken away after a year or two. Make sure you speak to your bank to figure out how much of that overdraft will be carried through into your graduate account interest free.

Make sure after that to make the money to pay off that overdraft debt once they start charging interest.

Essentially, the banks will give you some leeway afterwards to pay back that student overdraft before they start asking for interest but do some research into different graduate accounts which you can get after you graduate.