Okay so who is going to take the plunge and do it?
If you downgrade I would like to start a section based on user stats and issues that were involved.
I would like to hear about all the systems, if they started on 4.5, or if they did not(although i don't recommend it if you never had 4.5 on your system). and since there is no direct guide for it someone can view these comments and possibly put one together. although it should be as simple as placing the file on the SD and loading it through GW menu, I would really like to find out what systems can and cannot downgrade.
(also feel free to discuss that directly here because it will get backed up elsewhere and jumbled so please keep all posts topic related. )
Gateway's quote
2DS consoles currently get this error
confirmed downgrade without GW red card
tested by WayLouka
http://gbatemp.net/threads/gateway-downgrading.378195/#post-5269868
(didn't see your edit aha)
Downgraded an Animal Crossing NTSC-U 3DS XL from 9.2 to 4.2
USA models are being confirmed downgrading to 4.2 whereas others 4.5(with ULTRA still working)
PRE 4.5 models are downgrading fine