Hi @Sabo and thanks for posting.
I'm sorry to hear about your girlfriends Nan, please pass on my condolences. Is that account being kept on by another family member? If not then it will need to be ceased which will free up the number on that line. Provided it isn't taken by another customer you could request it once your line is up and running.
The number was actually used as an office (for farming business). My girlfriend (the granddaughter) wants to keep the number for business/convenience reasons.
I got that general idea from reading other posts. Would it likely to be more successful if I had the new account up and running beforehand, or cancel old and setup new at same time.
You could take a chance in cancelling the old account before setting up the new one but once the account is ceased the number will go back into circulation so there is a chance you could lose it. Your best bet would be to contact the business sales team on 0800 800 152 and they'll be able to advise on the best way forward.