When your site is indexed by google you could add this .htaccess lines which redirects the .html to without so the old pagerank don't get lost.
Options +FollowSymLinks -MultiViews
DirectorySlash Off
RewriteCond %{SCRIPT_FILENAME}/ -d
RewriteCond %{SCRIPT_FILENAME}.html !-f
RewriteRule [^/]$ %{REQUEST_URI}/ [R=301,L]
RewriteCond %{ENV:REDIRECT_STATUS} ^$
RewriteRule ^(.+)\.html$ /$1 [R=301,L]
RewriteCond %{SCRIPT_FILENAME}.html -f
RewriteRule [^/]$ %{REQUEST_URI}.html [QSA,L]
Evolution user, I like the back-end speed and simplicity
