Writeable clusters

I saw there are some clusters (for instance in the Basic page) that are RW.
I’m curious about the 0xFFFD
This is the Sonoff with FW 2.0

This is the “old” 1.0

What could happen if I modify that cluster in the old one from 1 to 2 ?

Ha, you have find an issue
From the documentation, this attribute (and not cluster) is write only.

Id Name Type Range Access Def M/O
0xfffd ClusterRevision uint16 0x0001 - 0xfffe Read Only 1 M