Those are just underline characters - which still work the same way as before. Note, too, that just typing three of them, then pressing the Enter key, will give a solid line from margin to margin. Your description of what happens now when you press the Enter key at the end of an existing line suggests that's what you already have. That line is actually a paragraph bottom border and can be removed by setting the paragraph border to 'none'.

Unless you can explain what you mean by "documents I have created in the past with that line are all messed up", we can only guess what the issue might be.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]