Does 301 redirect impacts the SEO ranking from Google stand point of view

This is a very critical used case where customer is migrating from 1 CMS solution to another, where all their pages do not have any .html extension. However their current CMS solution - all pages does have .html pages. We are thinking of placing a redirect for .html pages to current pages without extensions. Which is 301 redirect. 

As per our understanding - if its a 301 - means Google will start increase the ranking of the destination URL and start decreasing the ranking of the source URL. Which means after a period of time, anyway that Old source URL ranking with HTML pages may get nullified. Is that assumption correct ? Indeed post migration, new robots.txt and sitemap should be indexed. 

OR shall we make an effort to keep .html extension intact by what ever means - like keeping the URL intact with .html while getting the content from the new CSM solution so that we get a 200. 

Now wont that impact the SEO scoring of the site since we really want the old .html pages ranking to go down & moving UP the ranking of the new pages without .html. 

Please advise. 

Regards, 

Meet 

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