When Clockwork sends Danny into the past he is able to exist freely in both the past and alternate future without wearing a time medallion, even though in "The Ultimate Enemy" Clockwork explains that the only way to exist in a different time-line (that isn't your own) is by wearing one.
When Jack Fenton is first shown after Danny changes the time-line, he has his normal present-day hair-do, but when Jack transforms out of his ghost mode in front of Maddie near the end, his hair goes back to how it was when he was in college.
When Danny is first shown wearing '80s clothes, the cuffs on his sleeves are black. When the cuffs shown again, they're white.