Hi there
Now, I've not used that plugin before, but I have helped to successfully migrate a legion of 8 sites to a new CMS, in the process going from .HTML to .asp keeping the rankings in tact (actually improving a little with fresh content), so hopefully I can help.
We ended up 301 redirecting the old URLs to the new ones, which worked for us. It was made a hell of a lot easier by keeping the same link structure, which I see you're planning on doing. I can't stress this enough, it helps so much if you can do this and replicate the structure.
We ended up dropping a few pages in the migration. We asked two questions of these pages - a no to both resulted in letting it return a 404, which is OK (don't be afraid of returning a few). If the page was either a) bringing in traffic at the first point of entry or b) carrying some link "juice" from external links, the page was 301 redirected to the nearest equivalent page in the new structure.
Now, none of this involves that plugin - I'd be curious to see if anyone has used it. I like the idea of it if it effectively means your URLs stay exactly the same. However, I'm just here to say that 301 redirecting has worked for me in the past. We've read recently that a 301 will pass all the previous strength of the link, which is also some comfort.
As I said before, it was made infinitely easier by keeping a consistent URL structure. If you can do that, which you're aiming to do, minus the new extensions, it can be a quick and fairly painless process. If you want some advice on how to quickly get the 301 lists ready, let me know (hope you like excel!).
Hope my input helps, but I'm definitely joining you in wondering if anyone's used that plugin. Failing that, 301s can help preserve rankings.