You're right, when you delete your LJ you won't be able to log in to you OpenID account any more, and so wouldn't be able to edit those posts.

I believe that claiming your OpenID account with your Dreamwidth account would be a way to still allow you to edit, but I can't say for sure. I made the mistake of deleting my LJ before claiming OpenIDs was a thing, so those posts and comments are lost to me.