I think when you install a new version over the old one the old settings are kept (not 100% sure, though) unless you check the box in the installer to delete the old configuration settings. But of course it would be a good idea to rename the old version directory from, say, c:\lazarus to c:\lazarus-old, in order to keep it as a backup installation (just rename it back to have the old version again).
If your old installation was a "secondary installation" you should use the same name for config directory of the new installation.
After installation you will have to rebuild the IDE if you had installed additional components. For this purpose, go to "Tools" > "Configure Build Lazarus", check "Clean all" and "Switch after building to automatically", click "Build". After some time the IDE restart and you'll be ready to go.
Of course, these steps do not work when your additional components are installed in a subdirectory of the Lazarus installtion - sometimes, people recommend here to install third-party components in c:\lazarus\components. This is bad advice because it makes reinstallation of Lazarus unnecessarily complicated. Always install components out of the Lazarus installation folder.