Do not modify old urls?

Hi, I have in mind to move from phpbb to Xenforo, but I have some doubts.

The old url of the themes created in my forum, are like this:

Then I ask if these are not modified to the style that Xenforo has, I say something cleaner:

What I want to say is that I want to keep the old URLs in the old topics, and that the new topics have that style of Xenforo URL, cleaner or better for SEO.

It's possible?

I hope you can understand me, since I am using Google Translator.