As long as you keep your old CSS theme, switching back is easy. However, I recommend creating a test category (like testcss:page) within which you can test the CSS.
In order to convert to a Bootstrap layout, you need to install a new layout that uses Bootstrap classes. At the moment, there are two available:
- Wikidot's official version (pretty close to the typical layout)
- The Standard Layout, a community-created layout that tries to take advantage of a few Bootstrap features
You can select which layout you want in the Site Manager, under the Thems tab. The Custom tab contains the list of custom themes your site uses, but at the bottom it also contains a list of layouts. If you want to use the Standard Layout, then you must create a new layout and copy/paste the code from this page; Wikidot's layout doesn't require this step.
Then, to apply the layout, you need to make a custom theme. You can then find the "Choose Layout" option that allows you to select a custom layout for that theme. Some of this is detailed on this page.
Finally, you can apply your custom theme by going to the You tab and applying your theme to the testcss category (or to the entire site if you wish).
To revert back, you only need to change your settings back to the way they were.
I'm sorry that I cannot give much better instructions than this at the moment; I've been constantly pressed for time the past few months. Hopefully this can at least point you in the right direction. If you have any questions, be sure to ask.