Test Strategy for historical document content migration

Looking out for strategy for testing the document content migration. Here are the details.
We are re-writing a community sharing application where users can share views, articles, contents, images, etc as part of it, as part of it we are migrating the historical content using traditional data migration approach by applying transformation logic at needed places. - I am clear on testing this as its little straight forward of comparing data between source and destination with transformation logic.

But, having difficulty in testing the display of content when accessed through application to view online in the browser.
i.e. lets assume that a document with multiple content types like text, paragraph, images, slideshows, etc was created as part of legacy application. As part of new application, the content types are changed for couple of templates.

I could think of testing this by performing one to one comparison of document content and layout between legacy and new version of application. But this is time consuming considering the amount of migrated files content to be tested. Is there any better approach or automated solution to achieve this?

Any help is appreciated.

Thanks & Regards,
Basava Kumar M