Once it was confirmed that it was possible to go back to 3.2 or 3.3, I took the plunge and updated my 2019 MKZ Hybrid to Sync 3.4.19274. Took a couple of days to work out the kinks, some notes are below.
Vehicle: 2019 MKZ Hybrid Reserve II
If you follow the Cyanlabs instructions, it sounds like you only need to install the 3.4.19274 update if your already running 3.2 or above. This is only partially true as some files from the map updated are needed with 3.4.19274, so you can either update from the startup process (which will add a good hour to the time) or create a separate install for the missing packages and perform that (more information on which packages you need below) after or during the 19274 update.
Even with the map updates, there is a missing package from the normal Cyanlabs process. This is the updated voice file from 19274 and will prevent some Voice Navigation features from working. The missing file is 5U5T-14G391-CK_1571081000.TAR.GZ and can be added after the update and is for North America (US English, French Canadian and Spanish) only. I don't know what the alternate languages would be, or if they are available. You can add it by creating a new autoinstall.lst file and placing the file above in the SyncMyRide directory of your USB Stick.
Item1 = 5U5T-14G391-CK
Open1 = SyncMyRide\5U5T-14G391-CK_1571088381000.TAR.GZ
Options = AutoInstall
The APIM firmware updates are not required on the 2019 models (see one possible exception below) and are only required on the pre 2019 models if you want to add the Wireless Apps, Calm Screen and Radio Logos (not to be confused with Sirius XM logos and seem to be for Europe only). If you have a 2019 all should work. These are my non-updated firmware versions from the 2019 MKZ Hybrid Reserve II.
Secondary Boot Loader - GB5T-14G376-AA
Strategy - 1U5T-14G374-DA
Calibration - 1U5T-14G375-DA
ECU Configuration/Sound Profiles - GB5T-14G379-AA
ECU Configuration/Illumination Strategy - JB5T-14G379-BA
The only 2020 feature I did not see working was the album art in Sirius XM. I saw this working in a demo of 3.4 (https://youtu.be/wxjmmj4Wp0U?t=214) but it does not work on my car. I need to do some additional discovery to see if this is a APIM firmware update, a FORScan mod or an update to the GraceNote packages that are not present.
Before with 3.3 I had sporadic Sirius XM channel logos (maybe 1 out of 8 showed), but after the update, all stations in Sirius XM that I checked now have logos. A big improvement there.
With Android Auto, I now get the turn information and speed limits on the IPC (with the Hybrid, its the right IPC) when running Google Maps (but not Waze yet). With Apple CarPlay this does not seem to be working.
(YMMV) On my MKZ hybrid, I had it configured via FORScan to show the heated and air conditioned seats and steering wheel on the home screen. This is only partially working after the update. The heated steering wheel icon still appears, but not the seat icons. At times, a button appears (but is not able to be pressed) showing the icon for the Power Flow app, so this may be an issue specific to the hybrid models. On a side note, resetting the setting in FORScan to not show the seat and steering wheel buttons, now has the effect that Sync now remembers which screen I was on when I restart, so if I left off in Navigation or XM, that screen reappears when I restart the car. This may be present in previous versions, but I just noticed due to previous configurations. I like it better and will be keeping it this way since the physical seat buttons are always visible anyway.
Calm Screen works by enabling the setting in FORScan (7D0-02-03 - X4XX XX) and can be accessed by Settings -> Display and a large Calm Screen button now appears. It can be turned off by tapping the screen.
19274 adds lots of new Navigation commands that will not work without all the correct packages aligned. In fact, some previous navigation commands will not work without the correct voice files. If you go into the Bezel Diagnostics -> APIM -> IVSU Versions, you should have the following versions and can download and install any missing ones (or use from another package):
You can now also use apps (Waze, Pandora, IHeartRadio, Sirius XM) wirelessly (no longer USB tethered). Waze works this way with Apple, but not Android. I do not know if this is APIM firmware restricted, but I think it is.
My FORScan changes are all working as expected, with the exception of the APIM based one I used for the seat icons on the home screen.