A Witch in Time and Time Travel

In Forever Charmed, Future Piper who travelled to past and merged with her past self. And so did Future Billie. I guess there's really no point in traveling back to the future when your future is already a few hours away anyway. Unless of course there are people from both past and the future in your present which need to be returned to their respective times.

And Chris actually started to disappear since he messed up his conception when Leo became an Elder not to mention making Leo always suspicious of him after sending him off to Valhalla and being distracted by whatever Chris was up to.

In FC Piper merged with her past self because her past caught up with itself. She was present at the point when her past self would have used the ring to go to the future, but because the future self had already done that (and thus changed the... I want to say past, but at this point it's the present) they merged into one person because it was a paradox.

In the episode where Phoebe continually saves the life of miles. Piper used a time tunnel to go backward in time, then warns her younger self to go right instead of left, this leads to the original Piper no longer existing because the events that would lead up to her going back in time, no longer happen. Miles wouldn't be saved that day, and the next and so on, until around six months later would Phoebe die. The whole chain of events has been stopped.

So basically, in miles' case. Piper wiped out her future self from existing because she altered the event that would lead to her needing to go back in time in the first place.

While in FC, Piper(A) had already altered the timeline by recruiting grams and mom, when she returned to the present and merged with Piper(B) she had to merge, firstly because time had caught up with itself and secondly because if Piper(A) was erased (like miles' case) Then Piper(B) wouldn't have had the chance to recruit grams and mom. They would have dissapered because Piper(A) wouldn't have existed (because NOW Phoebe and Paige are alive with Piper(B). It's a bit confusing, but the angel of destiny tells Piper(A) that "the battle is over... though not as I expected" hinting that the end of KBV2 was wrong, she later confirms this to Piper(merged) at the end of FC when she said "this was how the battle was meant to be won" meaning that Piper was meant to use time travel to change events.