When you move to a different host, will the domain name also change? If it does, then you can change the file extensions too.
If the domain name is staying the same, then you can and should keep the same extensions even if the pages themselves use plain HTML or are generated by a PHP script.
There's very simple settings to enable PHP to scan .aspx files for PHP code (AddHandler etc).
The files on the server can have the .html extension while using .aspx in the URLs for those pages. That's a one-line simple rewrite using a RewriteRule.
However, if you are convinced you need to change the extensions, from aspx to something else, then consider completely removing the extensions from the URLs by going extensionless. The physical files on the server will still need an extension though.
If you merely change the extensions from aspx to html, I forsee you posting here in a couple of years time asking how to change from .html to .php extensions. Avoid all of that and go extensionless now.
The two RewriteRules needed for this (one configured as a redirect, the other as a rewrite) are beautifully simple.
There's three factors to consider here:
- what technology generates the page: "plain html in a text file" vs. "a scripting language such as php",
- what extension do the actual files "here" inside the server have,
- what do you want the URLs used "out there" on the web to look like?
You can configure any combination you want. That's what URL rewriting is for.