Fellowes explained that even if the 77-year-old actress were to leave the show, the character of Violet Crawley, the Dowager Countess of Grantham, will not pass away.
"I'm happy to say the Dowager is not going to die," he is quoted as saying in The Sun.
"Violet is going to live by the sea in case Maggie changes her mind."
Maggie Smith has been rumoured to be departing the ITV drama over the past year, with Fellowes previously stating that her exit would be "up to her".
Earlier this month, Smith was taken to hospital suffering from chest pains.
Downton Abbey was recently reported to be close to a fourth series renewal in the near future.
The ITV period drama's executive producer Gareth Neame revealed that the show's future should be confirmed "in a few days".
Watch Digital Spy chat to the cast below: