officialy yes, its not possible to downgrade.
But in the underground part of this forum you'll find a way to downgrade. it works with 571 too.
Note, you first have to install the new FW, then change the file VERSION, then install an older FW. And THEN you can delete the partitions and reinstall another FW from scratch. If you remove the partition BEFORE this procedure it wont work as you also delete the changed VERSION file.how to downgrade FW
*Please do not Private Message me for support questions; leave it on the forum so all members can learn. Thanks!*
DS713+ / DSM 5.2-5565-1 / 2 x HD204UI (RAID0) / LMS 7.7.3-042
DS712+ / DSM 5.2-5565-1 / 2 x HDS721010KLA330 (RAID0)
DS110+ / DSM 5.2-5565-1 / HD154UI
DS408 / DSM 4.0 2265 / 2 x ST31000340AS (RAID0) + 1 x HDS721010KLA330
DS207+ / DSM 3.1.1639 / ST31000340AS + WD5000KS
2 Squeezebox 3 + Boom
APC Smart UPS SUA750i