Adjusting IBUs

All-Grain recipe with a 60 min boil.
When adjusting a recipe for different AA %s, can you adjust the boiling time of the hops (say bittering from 60 min to 45 minutes, flavor from 30 min to 20 min etc) or is necessary to adjust the amounts used? Does it make a difference?

Yes, you can. It will make some difference in the taste, but perhaps for the better (more flavor and aroma). I'm a big fan of later addition hops as long as I get the bitterness I wanted. That said, I brew a lot of Pale and IPA.

What kind of beer are you brewing? Someone else will probably give you better advice based on the beer.

It is a Pale Ale - Hops: Magnum .5oz for 60 (or 45)
Pearle .5oz for 30 (or 20)
Cascade 1 oz for 10
Cascade 1 oz at flameout
Cascade 1 oz dry hop

I aim for getting most of my bittering from the bittering hops. 90 mins is max for getting all IBU's from the hops, I generally go about 75 mins and put the bittering hops in after first running, to get a good utilization.

Flavor at 20, and a bunch more at flameout for aroma. Very little IBU's come from the 20 addition, none from the 0 one.

Adjust amounts and boiling times to make changes from varying AA% of same hops than was in recipe, or to use an alternative hops choice.