Mudar de Ideia
Brazilians are a flexible bunch. They love to set dates, appointments, meetings and decisions, often without much hesitation. But it's all in a constant state of flux. You will discover that it's partly just their nature to agree to things. It's actually a very charming trait. But, it also causes a lot of energy to be spent on UNDOING what was agreed. To reduce the pain and suffering that this can bring, I jump through hoops trying to motivate Brazilians into committing. If we've set a date do do something - ANYTHING, I almost always call the day before to confirm. If it's a business appointment, I always call to confirm that I've confirmed (!). This is especially important to do if you're in Rio de Janeiro. They are the most dedicated mind-changers I've ever seen. If you don't call ahead to confirm a date, it's not unusual for people to just assume that it's not on.
Thus, one of the most important phrases you need to know is:
*Notice that MUDAR is in the past tense. It's just like in English how we say, "I changed my mind."
People usually just say, "Mudei de ideia" -- without the "eu". Some great examples on Reverso Context here.
For the advanced learner:
Mudar de Ideia!