Issue 1: Unless you are using the virtual folders option, the forum URL should correct itself. Keep in mind that phpBB caches SQL results and you may have to purge the forums/cache folder (see the first page of the ACP). You probably want to enable the "zero dupe" option to redirect from the old to the new URL.
Issue 2: By default, the .htaccess generated by phpBB-SEO includes support for all the URL types, but it doesn't hurt to double check by regenerating and comparing it against your current .htaccess. You don't need to purge the phpBB cache or the phpBB-SEO cache after changing the URL optimization type, though it can't hurt if it you do.
Issue 3: Same answer as #1 above.
PS: When in doubt, see How to set up a local server using xampp
and test your installation on a local test server. Then you can experiment without bringing down your live site.