Olen, hello.
Well part of this problem, which is super common, is that DJI's English instructions are, let's say, Chinglish instructions, at best. I'd guess a degree in English Language out of China isn't quite the same thing as an English degree out of Europe, let's just say.
Your 99% error comes from this:
You start reading the instructions for the upgrade, and following them, plug in your drone, etc., then you come to this little part, around halfway down, where it says that you'd better have previously upgraded your remote, otherwise the drone upgrade will not complete.
Thanks a lot, DJI. Putting Step 1 way the heck down there as an oh, by the way, instead of in front, like most other Step 1's go. [Come to think of it, I actually do speak Chinese. And even in Chinese, 1 comes before 2, usually. Go figure.]
So yes, you should have previously upgraded your remote software before you started upgrading your drone, silly. Everybody knows, you have to read everything down to the bottom before you start anything with DJI, right? Right...
No big deal. Just reboot your drone, and reboot your remote. After a couple times of disastrous things happening, mainly in the remote side - for instance, the usually green LED on the remote starts blinking all kinds of colors, signifying bad lightbridge (HD) signal, gimbal stutter, all that stuff... just ignore it all (don't post about it yet) and wait until one reboot suddenly realizes that your remote hasn't updated yet. It will ask you to do two things - one is to update your No Fly Zone databases, and the second will be to update your remote.
So update your remote controller, and then both of them (remote and drone) will be updated. Had you done your remote controller before your drone, that last 1% of the drone update was actually checking to see if the remote was updated. It wasn't, so your drone update program hung up at 99%, waiting to hear from the remote.
After both are updated, just reboot until everything clears up. No need to re-connect in my case (I've done two
P4P upgrades thus far), and eventually the HD issue clears up with the upgrade and the remote light goes back to green again.
Now you just have the regular upgrade issues (no more smart RTH, display lags, loss of signal happens closer than before) the rest of us are now having!
If/when you want to downgrade again, you should do it with your computer direct to the drone and remote, using the Assist program.
Since you've already had to check in with DJI for this upgrade, running the downgrade should restore full functionality and range as before. You'll still get the UPGRADE NOW!!! warning, but I think that DJI really shouldn't be stupid enough to re-brick your drone because you didn't attempt the upgrade. Lord knows, it already recorded everything else about your drone already, so it should know that you already upgraded, took the blue pill, and spat it out again.
I mean, they already have our information, what more do they need?
DJI, how about giving us a fully functional drone, like we paid for? Without all this nonsense?
LOL!!!