Extension rename to .aspx

No that is not possible (with WP), use 301 redirect rules to redirect old links to new.

Thanks Seacoast, renaming extensions to .html is possible if I am not mistaken, why is it not the same with .aspx ?

if I am not mistaken

You are.

For posts, you can do this in the Settings->Permalink settings. On the custom option, you can have something like /%postname%.aspx and have it work.

For pages, you’d need to have some custom coding to change the permalink rewrite to add the .aspx. I haven’t tried it so don’t have a good suggestion for you. There was an old, old plugin that changed page permalinks to html; however, there has been some reported issues with parent pages, so that probably isn’t the best route.

use 301 redirect rules to redirect old links to new.

this will be the best option, WP, plugins, themes, work poorly outside of basic permalnks

Seacoast is right though. Redirects will be best, even if it is actually possible to do otherwise via WP. If you ever migrate away from WordPress to, say, a PHP-based static site, having .aspx would cause headaches.

Might as well take care of it now.

thanks for the tips, I have advised the client against the benefits, some clients just won’t listen.

Brandon, thanks for that, tested that on a fresh installation and it works fine. Will figure our a workaround for pages.