I would suggest that you head up your non-grammar entries with your preference(s) for when you have moved, followed by at least one which would be your preference based on your current address.
That way, it would go:
Scenario 1: Move goes ahead, do not get grammar school: Get one of the prefs based on new address, be on waiting list for grammar, no need for lower preference, current address places.
Scenario 2: Move delayed, do not get grammar school: do not get prefs based on intended address, but go on waiting list for grammar and 'intended address' schools, get pref named based on current address.
In Scenario 2, as soon as you do move and confirm your new address, you (theoretically) move further up the waiting list for any of your 'new address' prefs.
If you list your 'current address' prefs first, on March 2nd you may or may not still get a 'current (now old) address' place, but you won't automatically be on the waiting list for your 'new address' prefs, as you ranked them lower. That being said, my observation would be that Essex CC admissions do try to do their best to get allocations based on a change of address out as soon as possible, once you explain what changes you want to make.
Of course, if you get the grammar place anyway, all this is rendered academic, so to speak

.