Actually, this update is fully backwards compatible with older browser versions as well as mobile / tablets, to ie6 level and upwards, AND, you will now have a functional WYSIWYG (What You See Is What You Get) editor in your mobile and tablet devices, not just a text field for typing like you have now. The current editor is old and pitiful and doesn't play nice on all browsers, let alone non-existant on mobile devices.
Server software has nothing to do with accessibility... it is purely the HTML/CSS you get at your browser. That is all coded for backwards compatibility. Sure, it may be ugly in places on really ancient browsers, but it will function and be viewable.
This is one purpose of the upgrade, especially with PTSD Forum... as this forum is already backwards compatible and nothing will really change here in that sense, other than the editor becomes available. PTSD Forum will become more accessible.