So I will share some photos, in order to help those thinking about a similar upgrade.
Feel free to ask questions (if any), I'll do my best to get you an answer!
First of all, the M9 gimbals have Hall effect sensors in them. These sensors work at 3.3v (2.5v to 3.5v according to the specs provided by FrSky) and will very likely fail if fed with 5v. This means that such an upgrade is easier to those who have AR9x or SKY9x or 9Xtreme boards installed (these boards already work at 3.3v). In fact, using M9 gimbals on the stock mainboard with some simple level converters will likely cause the loss of resolution, so I don't think this upgrade is feasible on the stock board. I personally will have AR9x fitted into this radio, so I'm all set for this upgrade. Some more details about the electrical connection will be added later.
Now, the physical installation.
Unlike "traditional" Taranis gimbals (PR10), the M9 requires some more mods in order to fit a 9x enclosure. The metal plate of M9 binds trim switches in the 9x (both horizontal and vertical) and doesn't let the gimbal to sit properly. The solution here is to cut some metal off the plate - so get your Dremel ready!
Photo below shows the areas where binding happens (on the right gimbal) - marked with red:
I was a bit too optimistic and started to cut and trim the plate without taking it off the gimbal...
It sort of worked, but there's a lot of dust and the metal gets very hot. So I found it's much better (and safer!) to remove the four screws holding the metal plate (see the very first photo) and then work on the metal plate alone. Here's the result:
After the plate is trimmed (and tested to fit the enclosure), put the gimbal back together.
Note that I didn't have to trim any other part of the M9. It's just the big metal plate that needed some corrections.
Now the trims buttons are out of the way, but ... the gimbal still won't fit:
There are stands in the enclosure that work well for the stock gimbals, but do not do anything good in case of M9:
A closer look:
We need to sand the upper (thinner) part off completely. I used my Dremel here, too.
Don't forget about the upper right stand. There's no thinner part there, so just cut it to about the same height as the other three.
Take care not to cut too much!!!
Now the gimbal fits nicely:
Here's a photo of my 9x with FrSky PR10 gimbal on the left and the M9 on the right:
The height of the stick itself is pretty much like the PR10. Of course, the height can be adjusted to your liking.
Inside the radio, the M9 gimbal is noticeably lower and shouldn't cause any issues to close the back cover.
A few words about electrical connection of the new gimbals.
I didn't want to solder the new wires in. Instead, I installed MW-3M connectors on the trimmer boards.
For the right gimbal:
For the left gimbal:
This allowed me to carefully pull the wires from the long connector it came with and put them into an MU-3F. No crimping is needed, the stock pins get into the new connector perfectly fine!
An important thing to remember: trimmer PCBs are made in a way so that the connectors can only be installed "mirrored" to each other. It means that the wires from each of the Hall sensors should be plugged into the 3-pin connectors differently: one should be red-brown-orange, and the other one should be orange-brown-red. Just be careful and give it some thought before you proceed.
Also, take care not to swap the vertical and horizontal sensors when you connect them to the trims PCB. Otherwise reverse polarity will likely damage the sensor.
