Yes there was a change in Tableau 7. You can achieve the same results in 6 by using a calculation like
Datetrunc('month',[your date field]) and setting it as exact date.
I cannot show you how to do this on your worksheet as you need to post a tbwx (save as) that includes the dataset.
I edited the original post and replaced the attachment with the packaged file. On Sheet 4, I used the exact date approach. Then I put Date on the level of detail - had to make it an attribute, or else you get the year breaks again. But when I try to put yearly reference lines in, I can only do min, max, or constant. So I can get a reference line for the first year, but that's it. So Sheet 4 is what I'm basically looking for, except with ref lines at each year's start.