As long as you install over the top of your existing settings, nothing much will be lost. You can turn your old toolbars back on afterwards if desired: Your old toolbars are still there after you upgrade.
Font sizes will be reset, as part of the new high DPI support, but you'll probably find you do not need to adjust those as Opus will automatically use larger (in pixels) fonts on high DPI machines now. If you do need to adjust font sizes, it's just a few clicks in Preferences / Display / Colors and Fonts for most of them.
You're using a custom toolbar so the buttons/commands on it must have come from somewhere. You can edit the button to find out what the command is like this:
- Select Settings > Customize Toolbars to go into toolbar editing mode.
- Where the list of favorites was on your toolbar, there should now be a single button which is what generates them. Right-click it and choose Edit.
If it runs the Favorites command, you'll want to add NOOPENINTABS to the line.
For example, if the old command was just Favorites on its own then the new one would be Favorites NOOPENINTABS
(Or if the old command was Favorites SHOWICONS USEQUALKEYS the new one would be Favorites SHOWICONS USEQUALKEYS NOOPENINTABS. And so on.)