Does a spell's converted mana cost change if you cast it using an alternative cost like Force of Will?

A spell's mana value (the term that replaced 'converted mana cost' in 2021) is always calculated from the mana symbols printed in its mana cost, regardless of what you actually paid to cast it. This is established in CR 202.3, which defines mana value based on the mana cost, not the payment method.

Alternative costs — like the one on Force of Will, which lets you exile a blue card and pay 1 life instead of the normal cost — change what you pay, but they do not change the spell's mana cost itself. CR 118.9 clarifies that alternative costs are a different thing from a spell's mana cost, and CR 202.3a confirms the mana value is derived solely from the printed mana symbols.

This matters in many practical situations: a spell's mana value is checked by cards like Spell Snare (counter a spell with mana value 2), Chalice of the Void, or Cascade. None of those care how you paid — only what the printed cost says.

Concrete example: You cast Force of Will itself (mana cost 3UU, mana value 5) using its own alternative cost (exile a blue card, pay 1 life). Its mana value is still 5 on the stack. If your opponent controls a Chalice of the Void with 5 charge counters, Force of Will will be countered, even though you paid zero mana for it.