How to change from Ubuntu touch stable channel to rc-proposed without loosing any data on the device? Is that possible without backup and restoring the data?
Install the Terminal app from the store (or if you have enabled developer mode, you can use the
phablet-shell tool over USB connection). If using Terminal app on the phone, you may need to disable screen locking Battery Settings first, as if the screen locks, the Terminal app (and its child processes) will be stopped. You can then change the setting back after switching channels.
Then, on the phone, run
sudo system-image-cli -vvvv --switch $newchannel where
$newchannel is the new channel you switch to switch to. For different devices, this channel may be different. For example, on the BQ Aquaris E4.5 and E5 Ubuntu Edition phones, the channel should be
ubuntu-touch/rc-proposed/bq-aquaris.en to get rc-proposed. You can use
system-image-cli -i to show current information, and generally just replace the
stable portion of the existing channel name with
rc-proposed to get the right channel. You can also perform the same action in reverse to switch back.
It is also possible to follow the instructions here, it appears to only change the system partition: https://developer.ubuntu.com/en/phone/devices/installing-ubuntu-for-devices/
I think these are the important steps (on an Ubuntu desktop):
Add the Ubuntu SDK Release PPA:
$ sudo add-apt-repository ppa:ubuntu-sdk-team/ppa
Update your system to use the latest packages:
$ sudo apt-get update
Install the ubuntu-device-flash package
$ sudo apt-get install ubuntu-device-flash
switch the channel, e.g. for the bq M10:
ubuntu-device-flash touch --channel=ubuntu-touch/rc/bq-aquaris-pd.en
Your tablet needs to be running, with developer mode enabled (and connected via USB to the desktop). You do not need to boot into recovery mode. If it doesn't work keep trying, it doesn't always work on the first attempt.