Solution:
You would like to remove .aspx
from old links. Have your .htaccess like this:
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{THE_REQUEST} \s/+(.*?/)?(?:index)?(.*?)\.aspx[\s?] [NC]
RewriteRule ^ /%1%2 [R=302,L,NE]
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress