Any easy way of upgrading from Mixed mod to Advanced mod?

Well, most of the code is shared, so you could proceed as if you where installing the advanced mod over your current files, you'll just find many pieces of code already in place.
Then you'd need to overwrite the phpbb_seo/phpbb_seo_class.php (making sure you re implement your eventual custom settings) file and generate a new .htaccess and voilà.

Additionally you'd need to take care about the required redirection between the two mods urls.
To do so, you'd need to install / activate the zero duplicate mod if not already done (or change to the advanced version), and keep the rewriterules from you mixed mod .htaccess that differs from your mixed ones at the bottom of your advanced .htaccess (in most cases for what you are doing only the mixed mode topic rewriterules).
Post here both your .htaccess if you want to make sure, testing is very easy, loading a mixed topic url should lead to an advanced one.

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@dcz: Thank you for this hints. I've already upgraded from mixed mod to advanced mod rewrite and followed your instructions. I've copied the ".htaccess" from the mixed mod to the new one of the advanced version and it work perfect. Now there is only one question: what about the "robots.txt". Should I keep the old one in place, or delete it, or put a new one in place? The zero duplicates mod is already installed.

Be aware that with phpBB2 you'll need to use the corresponding zero dupe.

For the robots.txt, if your site already was indexed using the natural urls, then wait at least two month before you implement it, if not, then implement it, it's the same for all the phpBB2 rewrite mod versions.