So, I just ran this test on a post more than seven days old. While I was able to add something to the post, I couldn't update due to there being more than 8 tags (I had published it Ecency, I believe). I deleted two tags and updated the post. My change was there, but the two tags I deleted reappeared. I went back and deleted my addition along with those two extra tags and the same thing happened. The two tags reappeared.
That tells me that you can edit a post, but you can't edit tags and other metadata. Again, that's where WordPress has an advantage.