Importing posts, preserving YouTube embeds

I’ve imported WordPress posts from my old site to my new site. The import worked fine, except for one point: YouTube embed code was stripped from my posts.

Is there a way to preserve that YouTube embed code when I perform the embed? In performing a little bit of research online, some people suggested that the solution would be to edit the kses.php file in the wpincludes folder to “comment out” the Post Filtering code. Is that what I do? And, how do I comment out something? Just add a hashtag?

Thank you!