Hi louib (or even luoib!
I think the answer to your dilemma lies in the Allocation profiles for the last 2 years.
In both 2006 & 2007 the Amersham school was able to offer places to all children who wanted it, regardless of their catchment. (So, by the by, was Chesham High.)
Therefore even if you put Chesham High first, he passes and you change your mind you can ask to be re-allocated the Amersham school and will almost certainly get a place. If you needed to appeal I think you would be on strong ground to say "my son has decided that he would rather be top in an Upper School than bottom in a GS, and he likes the Amersham school more than CHS".
However, if you put the Amersham first and he passes, you will not be automatically offered a place at CHS. You would need to appeal and as you rightly say, that could be tricky, having put an Upper school as first preference.
On guesswork, if you are in catchment for CHS but not for the Amersham you would be very comfortably within any likely distance cut-off for out-of-area applications that might be imposed for the Amersham if it were over-subscribed in future.
Please note that I am only working on historical information here and the situation could change in future, but I think it is unlikely. "Past performance is no guarantee" and all that!
Also, bear in mind that Chesham High also has a new Head, so the ethos and atmosphere will be changing pretty radically in both schools over the next few years.
P.S. If you register on the Forum I can send you a PM. But you need to decide on that user-name first!!