3.2 comes with canonical URL meta tag in overall_header.html it sets an 301 for all your URL's. at least google treats it this way. Make sure it is enabled.
Couple things I found- 1) When Enable URL Rewriting: in server settigns is checked, user avatar on articles/news show broken. 2)Left side bar link to articles/forum post show a double slash after forum directory so site.com/phpbb3//viewtopic.php=etc URL rewriting or not. probably not a big deal but ...
Prosilver. Font Awesome seems to be 'flickering' like it's constantly loading when I navigate through my test board. I don't see this too much here or on other phpbb boards using 3.2 even if they are hosted on the other side of the globe. I tried adding some caching from htcaccess... still see the F...
Not a fan of the recent changes of bulky icons here but I think that's mostly because of the outdated theme of this site. Looks way better on Prosilver. Other than that, I like the feel and speed of 3.2.x so far.
...but as JimA said, the forum will not carry over the old seo URL's. Unfortunatly this is a very BIG problem. My forum has about 10.000 indexed pages and very well positioned in the search engines; changing the url inevitably lose the positioning and it would be a very BIG disaster! :o :o 3.1 sets...