...
In the end the solution was quite simple and it even is much easier than installing Braiins from SD-card.
First I used
ASICseer to remove the signature check, and immediately after this I used the
Braiins OS+ Web Package to upgrade to Braiins from the web interface.
No need to flash older Bitmain firmware first, and no need to take the miner apart and move a jumper back and forth.
Well the Asicseer install does a LOT more, than instead, just installing an old BM firmware that doesn't have the SSH lockout.
Ass firmware also, as is obvious, violates the cgminer license.
It also tracks your miner back to them (and installs various other software also ...)
So ... you'd be best to just do the simple BM downgrade to a non-SSH lockout version, then install braiins
... and stay away from the Ass firmware since that could be problematic.
P.S. as usual the mod here merits a post that suggests (temporarily) using a firmware that violates the cgminer license ...