If you have migrated SharePoint 2013 on-prem or older version of SharePoint to SharePoint online. You always want to change the Team site home page to modern home page.

Answer is, you cannot upgrade or covert old Team site to communication site, but there is way where you can change the old team site home page to modern home page.

  1. Open page library of the SharePoint site and save the home page on your desktop.
  2. Open downloaded page through notepad.

  3. Find :  <mso:PageLayoutType msdt:dt=”string”>Article</mso:PageLayoutType>

  4. Replace >Article< with Home
  5. Open the SharePoint site on SharePoint Designer
  6. Click on Site pages, Right click and create new .aspx page
  7. Name this page as home and edit in advanced mode
  8. delete everything and paste the code from notepad
  9. Save the page and set as a Home page

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I think this might help you with change the old team site home pages.