In the irregular past tense causal verbs lesson (http://www.textfugu.com/season-5/dict-past/6-5/) I learned that いきます would conjugate to いった.

So I don’t understand this example sentence from the “If” lesson (http://www.textfugu.com/season-5/tara/7-7):

えいが　を　み　に　いきたかったら　”Cineplex”　に　いきたい　です。
If we go to see a movie, I want to go to “Cineplex.”

According to the lesson on how to say going to, you conjugate いきます as normal, and according to the “If” lesson that means conjugating verbs to their past tense casual form. So why does いきます become いきたかったら?

I would have thought it would become いったら.

Thanks for the help!