Learn Finnish: Tämä on suuri muutos, joka vaatii aikaa. - This is a big change that requires time.

The lesson's rule about using the illative (like viikkoon, 'in a week') applies to negative sentences to express how long something has not happened. This is an affirmative sentence! Here, aikaa is simply the object of the verb vaatia (to require). Because 'time' is an abstract, uncountable concept in this context, the object must be in the partitive case.

Could I use mikä instead of joka to mean 'that' in this sentence?

No, you need to use joka here. In Finnish, joka refers back to a specific preceding noun (in this case, suuri muutos). Mikä is only used when referring back to a whole preceding sentence, a superlative, or an indefinite pronoun like kaikki (everything).

Is the noun muutos (change) related to the verb muuttaa (to change)?

Yes! Adding the suffix -os or -ös to a verb stem is a common way in Finnish to create a noun that represents the result of the action. Another great B1 example is kääntää (to translate) becoming käännös (a translation).

Does the verb vaatia (to require) always take a partitive object like aikaa?