The message Change Drum Soon indicates the drum unit will need to be replaced soon.
Even when the LCD displays the message Change Drum Soon, you can still continue to print. You can replace the drum unit when you feel the print quality is no longer acceptable.
The Brother machine has separate consumables the toner cartridge and the drum unit. Verify you replace only the drum unit and not the toner cartridge. If you want to know their difference, refer to "What is the difference between the toner cartridge and the drum unit?"