20th level Wizard spells

[DracoSuave]

When replacing L1 spells with L15 spells, you have to replace all 3.
However (PHB p28), when choosing a power, you can always choose a power of a lower level.

So ... you can replace sleep, flaming sphere, acid arrow with prismatic beams, wall of ice, sleep

Right. exactly. If fact, you could replace sleep, flaming sphere, acid arrow with sleep, flaming sphere, acid arrow. So to properly answer the original poster's question, think of what you have in your book as tiers.

Here's a handy list to help you, extrapolated from p 143 of the DMG.

Level 20 Paragon Path Daily Attack Power (1 options)
Level 19 Class Daily Attack Power (3 options, choose 1)
Level 17 Class Encounter Attack Power (1 option)
Level 16 Class Utility Power (2 options, choose 1)
Level 15 Class Daily Attack Power (3 options, choose 1)
Level 13 Class Encounter Attack Power (1 option)
Level 12 Paragon Path Utility Power (1 option)
Level 11 Paragon Path Encounter Attack Power (1 option)
Level 10 Class Utility Power (2 options, choose 1)
Level 9 Class Daily Attack Power (3 options, choose 1)
Level 7 Class Encounter Attack Power (1 option)
Level 6 Class Utility Power (2 options, choose 1)
Level 2 Class Utility Power (2 options, choose 1)


Don't try to lump all the powers together into one mass of powers. Seperate it out per level that you get, according to the table in the DMG for higher level characters. Don't make it harder than it has to be for yourself.

For the math:
9 total utility powers available, 5 prepared
10 total daily attack powers available, 4 prepared
4 total encounter attack powers available, 4 prepared

That's a total of 23 powers available as options, but only 13 of them are a-go-go at any given time.