What's the reason for needing to downgrade? It's not going to change the region of the cameras.
Could the downgrade have stopped because you didn't have the correct firmware? What was the source?
The stock firmware for the 3310 normal and the 3310 B version is here, sections 08 and 09 :
海康威视——以视频为核心的物联网解决方案和数据运营服务提供商
On other series, above 5.3.0 prevents downgrading to a lower version, though not always when the
tftp updater is used.