I have just joined this forum, and I notice that you posted this inquiry a while ago.
Since the range of the horn is so wide, players will always have a “shift register” with which to contend. This shift register is determined to a great extent by the physiology of the teeth and lips. If a player’s upper lip is thicker than their lower lip and they have a typical “overbite” their embouchure will have a classic “downstream” angle with upper-lip overlap. If one’s upper lip is thinner than the lower, and the player has an “underbite”, where the lower teeth sit further forward than the upper teeth, this will usually lead to an “einsetzen” embouchure and straight-out or even “upstream” angle.
Because of the many variations of physiologies of lips and teeth, the range on the horn where the shift register sits and what we do with our lips for this transition varies greatly from player to player. Whether for the low register setting the lower lip “rolls out” as you have described, or both lips become more puckered, or the jaw is dropped, 2 factors remain common: 1) Avoid any lateral, or sideways shifting of the jaw; and 2) All embouchure shifts can be made more workable through specifically-targeted exercises that deal with this issue.
In my book of studies called “Practical Exercises for Horn” I deal specifically with various physical aspects of horn playing, and in the section entitled “Facilitating the Embouchure Shift” I deal precisely with this issue. There is more information about these studies on my website:
http://maestro.idx.com.au on the page “Publications and Teaching”. This volume is published by 3-C Musikverlag in Bochum, Germany and has previously been available at the Juilliard Bookstore and at Patelson’s (New York). However you may be able to acquire it directly from the publisher Christoph Schieri at 3-C, whose details are on the same webpage.
I have used these exercises myself over many years, and have utilised them with great success for my own students, as well as for students of other teachers who have invited me to give master classes at various conservatories in the US. I believe they may also be of assistance in your case.
All the best,
Lee Bracegirdle