[Ehlaelt]

Means following that only current Wizard or Ranger class may cast 'ice wind' spell and you'll be not able to cast it after reclass to other class?

REQUIREMENTS
 * You must be a Wizard or Ranger in order to make use of this spell.
 * Casting this spell requires the use of 1 air reagent, 1 ethereal 
   reagent and 2 water reagents.
 * In order to practice this spell you must have achieved at least 60%
   Spell Complexity Comprehension.
 * You may not use this spell while underwater.

[Renee]

If you're a wizard or a ranger and you multiclass, you will still be able to cast this spell (provided that you've leveled enough to get your spell comprehension complexity back up to where it needs to be). You still maintain all the attributes and skills/spells from that class, and just couple it with your new class.

The only time that "must be [class_here]" applies and must be your CURRENT class is for marks. Otherwise, if you were once that class and multiclassed, you can still cast the spells, use the skills, or wear the items associated with that class.