Well, if most of the processing power stays on the main chain, then the fork would have lower difficulty.
There is a somewhat of a "Highlander" effect here, i.e. "There can only be one". The coin with the most hashing doesn't have to worry about massive swings in hashing support. There is no "spare" hashing available to instant divert to hashing that coin.
Each coin effectively pays (coin market value) * (block reward) / (hashes per block) to the miners for hashing. This means you can rank coins from most profitable to least profitable.
A profit maximizing miner would direct 100% of his hashing power to the coin with the highest rating. If all miners think the same, then the coin gets a surge in hashing.
However, if you drop from 1st place to 2nd place, you would get a massive drop in hashing, since all profit maximizing miners would switch away. This is one of the reasons that alt coins try to change difficulty slowly. They don't want to drop from 1st to 10th place and then lose 90% of the hashing support.
For the question at hand, you would need to estimate the scrypt support for the scrypt fork.
For example,
- All blocks below 250000 will use sha256 POW
- sha256 difficulty is doubled between 249999 and 250000
- scrypt difficulty is set to lowest at 250000
- All blocks between 250000 and 259999 can use either POW
- scrypt difficulty is halved between 259999 and 260000
- All blocks from 260000 will use scrypt POW
You would have different scrypt and sha256 difficulty targets. The target is for each type of block to occur every 20 mins during the transition. It is probably worth having a faster scrypt rise for the transition, since 10k blocks is only 5 re-target periods.