Actually i've just tested and there is no way to modify the filter rule. The option to edit the rule is grayed out.
I'm using 9.4 0715 so most probably the edit option is disabled due to the fact that it doesn't work.
Let's see if someone else has something to add
Interesting..... We're on 9.3.2 and I'm experiencing the opposite. Our default ( not custom ) rules/filters are getting corrupted ( We think from failing over ESM from Primary to Redundant/Redundant to Primary). We just re-upload the rules file and 15 minutes we are good again. note: this also prevents the autolearn function on the receivers.
I need to poke more at our 9.4.0 dev/test environment...