The Hexar Silent mode is nice, but it's not the end of the world if your Hexar doesn't have it.
That said, I've done the upgrade, and it does work on my Hexar Silver. Being an inquisitive kind of person, I also played around a bit and managed to cancel all the extra features one by one, using different settings instead of D[FE]. If memory serves correctly, it was:
= D[FE] is the full feature set.
= D[FA] is without autobracketing.
= D[F8] is without silent mode.
= D[F2] is without manual flash guide numbers.
After that, I enabled the whole lot again with D[FE].
What is crucial though is to make sure that before you change the D[] part, you've got to have the A[] set to A[00]. It appears that A[] selects an address in the Hexar's memory, and D[] sets a value in that address. Change a value in any other memory address and you may cause irrevocable havoc.
However, if you still don't feel courageous enough, then why bother? The Hexar Silver as it is is already about as good as it gets. Just leave it like it is, and have fun with it..