I started the firmware update and was getting the da da da da sound like I expected but now it has changed to a solid non beeping sound. Is this normal? What should I do? I don't want to turn it off and mess it up but how long should I wait

A long beep sound indicate that the upgrade is failed, try upgrade again.

How long has it been? my update was fast maybe 10min, did you see the gimbal re-boot? you can take the sd card out of the camera power it off first  and see if in the TXT file a success of failed .

Ok. I turned craft off and updated tx then powered everything on and all is good! Both controllers, 6 batteries, and the bird. Im not sure what was going on and was scared to turn it off in that state, but apparently the reboot finished the job

Have just been through the firmware update and unlike the previous one I had problems. Followed the firmware update guide (not the video as its not the same).

Failed to correctly update and essentially the bird was flightless. The usual dddd to d-dd was replaced with a myriad of different versions of sounds in pitch and speed.

Contacted DJI online support and they suggested formatting the SD and trying again. Did so but again no luck. Tried another SD, formatted it and ran the update again as per the manual (pdf). Did take nearly twenty minutes and went through an automatic restart which it again carried out a further update, I suspect it was for the gimbal as it was a lot more active this time. Another 8 ish minutes and d-dd was back with the rear light doing it bit.
Lesson here BE PATIENT

Controller was then updated via USB to the camera. All was AOK. Just required compass calibration.

A quick nervous test flight went a treat and loved the auto landing gear up on launch (at last).

I will say one thing make sure that your SD is formatted as exFAT not as NTFS as it will do nothing. Ps that wasn't my problem, believe it was the SD card.
Happy skies