After all, the real challenge would be to transfer the Exposure Notification data. Especially since he switched because of the Covid-19 app.
I also have an FP3 with LOS right now (17.1 GSI), but without being able to transfer the collected data of Exposure Notification I can’t go back to /e/.
There has been a thread (and an answer of the creator of microG) concerning the problem, but it is so vague that I am not willing to try it at the moment:
https://community.e.foundation/t/microg-what-you-need-to-know-a-conversation-with-its-developer-marvin-wissfeld/22700/25
If someone was brave enough to try, I would be interested in detailed instructions 