Unfortunately, to my knowledge, there is no ‘easy way’ to do this. That said you shouldn’t have to redesign your site from scratch and would get way with building a custom WordPress Theme yourself.
As I’m sure you’re aware a WordPress theme is built upon a series of .php files. The process of developing a theme tends to start with static HTML / CSS files which are then used to create the various .php files. With this in mind you should be able to use your existing .html files to create .php versions simply by injecting WordPress (PHP) code into the relevant areas. This in itself is a huge task, depending on experience. Ideally you need a knowledge of PHP and Theme Development for WordPress – This is a good place to start
Regarding the updating of WordPress. The WordPress Core and Themes are separate from one another meaning that there can be an update for either WordPress or a Theme. If there is an update for WordPress this won’t affect the Theme in any way+.
However if you are using a pre-made theme and have amended it in anyway, once you update the Theme you will lose all your changes. That’s where Child Themes come into play. A Child Theme allows you to amend the WordPress Theme indirectly, meaning that if there is a Theme update your Child Theme will remain the same i.e. all your changes will be intact. This is a good choice if you want to make minor changes to a Theme however you would be better off creating a new one entirely, just because you’ll be overwriting a lot of existing code (just my opinion)
+In large updates some functions could potentially become depreciated meaning they no longer work so this is something worth keeping in mind when updating WordPress