I played trumpet from 5th grade until 10th grade, when my high school band instructor asked if any of the brass players were interested in switching. We had an abundance of trumpet players, and a shortage of French horn players (as is often the case). I decided it might be fun to learn a new instrument, and so I made the switch. I would agree with the ratings of 7 and 9 that you mentioned.
The hardest part about making the switch to French horn from trumpet is the embouchure (the position of your lips/mouth on the mouthpiece). You know how on the trumpet, there are some notes that are fingered the same, and your embouchure determines the pitch? (For example, middle C, G, high C, high E, etc. are all fingered the same on the trumpet - no valves. Also, F and Bb are fingered the same - 1st valve down.) Well, on the French horn, there are EVEN MORE of these notes that are fingered the same. (For example, middle C, E, G, high C, E, F, G, etc are all fingered the same. Also, D, F, Bb are fingered the same - 1st valve down.) This means that the control over your embouchure must become more refined so that the right pitch comes out of the instrument when you try to play a note. On the positive side, though, many of the fingerings on the French horn are the same on the trumpet, so you won't have to totally relearn ALL your fingerings -- just a few of them!
Do you know whether the instrument you will be using is a single French horn or a double French horn? Some band instructors start their beginner students on single French horns; however, many people think it is easier to start out on a double French horn rather than trying to make the switch later on. (I would agree.) Double French horns have lots of extra tubing, and a thumb valve which changes your instrument to a Bb instrument. Using the Bb side gives you a whole new set of fingering to learn, unfortunately, but it also allows you to play higher notes more in tune than if you were using the F side of the horn. Normally, the French horn player uses the F side of the horn (and its fingerings) for (most of) the notes below Ab (the one above middle C). The notes Ab and above are playing using the Bb side of the horn and its fingerings.
Another challenge to consider: The French horn is an F instrument, while the trumpet is a Bb instrument, meaning that while the trumpet is just a whole step away from concert pitch, the French horn is a perfect fifth away. In other words, when you play a C on your trumpet, the pitch that comes out is a Bb. When you play a C on the French horn, the pitch that comes out is an F. I guess what I'm trying to say is that it might take a little time for your ear to figure out where on earth you are when you play a note. It's disorienting at first, but with some patience and practice your ear/head will adjust!
Another consideration: sometimes individual pitches need to be tuned using your right hand inside the bell of the horn. Your band instructor or private lesson instructor should show you the proper technique for holding your hand in the bell and for tuning individual notes. If you have a good ear, it won't take much for your ear to notice when you have individual notes out-of-tune and adjust them.
Are you in band or orchestra? The French horn often has beautiful soaring melodies to play in both band and orchestra. Unfortunately, if your band instructor often has the band play marches (think Sousa), the French horn is often stuck playing the off-beats (i.e., the PAH's of the OOM-PAH's). I hope you are good at counting and playing off-beats! lol
I hope that all this information does not discourage you from making the switch! The French horn is very fun instrument to play and has a beautiful mellow sound. If you are a good trumpet player and overall musician, I would encourage you to make the switch! I'm glad I did -- French horns are way better. ;)