Edge 820 Xert data fields... gone

Hi everybody,

I just downgraded my Edge 820 to firmware 11.20. I’m not used to do these things but from firmware 12.xx on, the device is not longer usable: unbelievable battery drain.

So I installed 11.20 and I had my battery life back (sounds awful).

But I cannot install my beloved Xert data fields back, since they ask for a firmware update up to 12.50… very sad.

Any workaround?