The default toolbars are turned on, and any old toolbars turned off (but not deleted), but I don't think any layouts are changed. ("Layout" has a specific meaning in Opus.)
The old toolbars can be turned back on trivially if needed. We assume that anyone who has done heavy customization will be able to do that without problems. On the other hand, less technical users would miss out on the hundreds of improvements and new features added to Opus 10's default toolbars unless we switched them by default.
The change only happens once, when you upgrade from 9 to 10; subsequent updates leave your toolbars as they are.
The list of toolbars which are turned on is stored in /dopusdata/ConfigFiles/toolbars.oxc, so in a corporate environment you can manage that file however you wish. (e.g. Copy and restore it either side of running the installer.)
The old default toolbars in Opus 9 dated back to Opus 6, with not much being added or changed (a little, but not much) over the years. With Opus 10 we decided too much functionality was hidden by default and it was time to do something about that, and we also reduced the reliance on three-buttons (favouring menu-buttons instead) because time had showed us that people didn't discover the right- and middle-button actions on them, missing out on even more built-in functionality.