In the iOS app, changing daily goals for 'the Coach', or even turning 'Coach' off, did not affect my streak.

Not really sure BUT I think that with Goals selected you will loose your streak IF you fail to reach your goal during the 24 hour period. Normally you only have to earn 1 XP to keep your streak.

As long as you are confident on reaching your goal, then there should be no problem.

This will not affect your streak. Today I lowered my daily goal because it was very high, and my streak is in tack. (This question/previous answers were from 3 years ago, so I thought I would give an updated response- I was terrified that I would lose my streak if I changed my goal, but luckily nothing happened).