So, I have performed the migration on DEV - there were some problems with it going out of memory, so I had to tweak it a bit (please have a look at D1059, that is what I ended up using by hot-fixing on DEV).

There still is a slight problem, though - the migration of DEV took about 12 hours total, which is a bit unreasonable. Most of the time was spent in `alembic/versions/dbfab576c81_change_schema_to_v2_0_step_2.py` lines 84-93 in D1059. The code takes about 5 seconds to change 1k results. That would mean at least 15 hours of downtime on PROD, and that, I think is unreal...

And since I don't know how to make it faster (tips are most welcomed), I suggest that we archive most of the data in STG/PROD before we go forward with the migration. I'd make a complete backup, and deleted all but the data from the last 3 months (or any other reasonable time span).