All is pretty much answered in the announcement (and changed files): http://community.mybboard.net/showthread.php?tid=16273&pid=107476#pid107476
As for templates, if the announcement does not clear it up:
None of your templates are modified or deleted. We re-insert our "base" templates which are the master copy for your forum - this means all templates that you haven't edited already use the updated version. Any that you have edited and were changed in 1.2.3 will show up under "Find Updated" in the templates section - from there it is your responsibility to either revert your custom templates or edit them to be up to date with the changes.
Now - templates listed in the announcement above with revert required require that some changes be made for the functionality of that particular feature to still work. Templates without the "required revert" that you have customised will work fine as they are - no need to revert or modify them.
Hopefully that clears it up.