Effect of recreating a website on SEO

I will be redesigning a site for one of my client and will be doing Local seo for him. Now his current website is created in asp.net and the url of the pages are created by passing parameters like this www domainname com/page.aspx?category=category1

Since I will be recreating the site in wordpress this will create a lot of 404 errors

Is it going to be harmful for the seo of the website that all the existing pages will not be available

Kindly let me know how should i approach this

Not if you redirect all the old pages to the new ones.

Just setup redirect from old pages to new

In addition to using URL redirects I would recommend creating a sitemap as well. Generally speaking Google is pretty good at picking up changes and once they crawl all your pages they should realize where the content has moved.