I believe it would be wise to switch the -a and -A command-line options for eclean-kernel for two reasons:
1) To be consistent with emerge (I just removed all my kernels because I used -a instead of -A)
2) Typing uppercase A is "harder" which would make it more like a "confirm to remove all kernels"
To not make current users confused it would probably be better to use uppercase R (-R) to remove all kernels and issue a deprecated message for -A. At least that is my opinion.