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Your second code sample using RedirectPermanent will do exactly what you want.
The mod_rewrite code sample you show will redirect all pages to their equivalents on the new domain as shown. You can omit the [QSA] unless you want to add *more* parameters to the query string.
Using the mod_rewrite method, you need to use two directives, just like you use two directives for RedirectPermanent. First, take care of the exception (welcome page), and then redirect the rest to their equivalent pages in the new domain.
Either method will work for your needs. Have a look at mod_alias and mod_rewrite in the Apache modules documentation [httpd.apache.org].
One note: Unless you have a definite reason not to do it, I'd strongly recommend dumping "index.html" and just redirecting "welcome" to http://mynewdomain.com/
Let the server map "/" to index.html. This makes it much easier to change to .php or .shtml pages (just for examples) in the future, because none of your incoming links will have to change. Also, note that most popular site URLs do not have /index.html on the end. See Apache mod_dir for more info on defining the default index page name.