Some external permalinks to site (from FB or search) not resolving.

Site URL: http://musicfog.com

We migrated a ten year old site from SS5 to SS7 Bedford.   Went smoothly, but now SOME of our external blog post permalinks are resolving to 404. 

Doesn't matter how old, it's unpredictable.  All the unresolved links are formatted as [blog link].html.  The .html extension seems to be the problem.  If I remove the extension the page resolves.   Is there any way for SS to treat links ending in .html (their old SS5 default) the same as ones that don't? (It seems to already be doing this for many links).

Here's one example -from an old FB post:

http://musicfog.com/home/2020/9/21/noel-mckay-the-old-pawnee-waltz.html

And here's the link that works:

http://musicfog.com/home/2020/9/21/noel-mckay-the-old-pawnee-waltz

The easiest and best thing you can do is change hits to  your 404 page to go to your home page or any page on your site for that matter including the main blog page.  This can be done in settings/design "404 page not found." There is a drop down menu to select a page. I don't think you want to go back and edit all the old Facebook links. 

Thanks for the suggestion.  Am more concerned that SS never mentioned the "upgrade" would kill 10 years worth of links.   It's not just FB, it's google search, ios links etc.   It's a problem.  Glad the SS5 site isn't dead yet - may have to revert. 

So turns out it's an easy fix -- found via extensive google search (SS helpdesk was unresponsive).  301 redirect seems to have fixed it. General instructions here:  https://support.squarespace.com/hc/en-us/articles/205815308-URL-redirects.   Specific code to strip .html from external permalink and redirect to new SS url was this.  

/[name].html -> /[name] 301