Rewriting categories URL; Will it make future problems?

You mean SEO? Compared to other forum software, Discourse does a good job with following proper SEO url practices out-the-box. It looks like you’re trying to make the url shorter but in the end, google’s algorithms look for the following:

  1. Original Content.
  2. Trust & Identity.
  3. Authority of the site’s subject.
  4. Visitor receptivity (low bounce rate; links elsewhere that point to your site, ect.)

You should be “worried” more about the content on your pages than the technical semantics.