The update won't even cause a fork.
I think what you mean is that coins from pre-fork will work on both old and new chains.
The difficulty drop-off problem is a problem that has plagued all alt coins.
Bitcoin is a "price maker". This means that when they drop the price they pay per hash, the market adjusts to their new price.
Some miners might switch to alt chains, but mostly they can absorb price shifts.
In addition, since they have such a large share of the hashing power, it is less likely that they get hit with a surge of hashing power.
Alt coins are "price takers". This means that they can't control the price of hashing power.
If they drop their price by a large step, then they will get no hashing power.
Changing the difficulty every block, even if the window is large is better than changing it in one big step.
If you limited the change to 1% per block, then after 144 blocks, you get a 4.19X change, which is pretty close to the current limits.
I suggest your new rules should be
- reset the difficulty to 1 starting at block <next block>
- the next 144 blocks have no block reward (other than tx fees)
- updates the difficulty every block based on the last 144 blocks
- caps the difficulty change to 1% per block
The 144 zero payment blocks prevent a "pre-mine" situation.
It gives the network a chance to find the current hashing power without it being a giveaway.
It also means that nobody can make the accusation that those who happened to be online when the fork went live got an advantage.