Your differentials are a bit like a testing ground, so you would reduce cyl in your diffs before reducing cyl in your norms. But keep the balance betweent he eyes the same in diffs and in norms.
If you have Right Eye -4.0 sph -1.5 cyl and Left Eye -3.50 sph -2.00 cyl
then you could (if you wanted) decide to get differentials that were 1.0 sph lower and 1.0 cyl lower.
Since 1 diopter of cyl is similar to 0.5 diopters of sph, that would give you approximately a 1.5 reduction, with would be a reasonable choice for differential.
That would leave you with diffs that were Right Eye -3.0 sph -0.5 cyl and Left Eye 2.50 sph and -1.0 cyl.
But in your norms, you would still have all your cyl.
Then after having these differentials for 4 to 6 weeks (assuming they feel good, and give you just the right slight amount of blur), you would then change your normalised. You could wait longer than 4 to 6 weeks if you wanted as well.
The only time I know of when you would be not waiting the 4- 6 weeks minimum between changing diff and norms, would be if you were changing something about the complexity, like if you were equalising some sph or some cyl. Then you would change both diffs and norms at the same time.
So you wouldn’t want diffs that were
Right Eye -3.0 sph -0.5 cyl and Left Eye 2.50 sph and -1.0 cyl (unequal cyl)
while wearing norms that were
Right Eye -4.0 sph -1.5 cyl and Left Eye 3.50 sph and -1.5 cyl. (equal cyl).
I hope somewhere in my big ramble your question got answered.