If I take off with good GPS lock up, the Home Point should be a good one in Phantom's memory.  What if, during the flight, it loses GPS (it happens when video taking is on) and then regains GPS later (after I switching off the video taking, or it drifts to a new location with a better condition), will the original HP (on take-off) still applies, or will it forget the previous memory and pick up where it regains the GPS as the new HP?  It has long been a puzzle to me.

Simply put, does home point change after GPS lost and regained during the flight?  If anybody has experienced this and has the answer, please share.

The flight control system will record the home point when you have 6+ satellites. This only happens once. And, it's also the reason you should never take off until after the home point has been marked.

"does home point change after GPS lost and regained during the flight" the simple answer is NO

Thanks to the answers, it is very clear to me now.

Further on this, I learned that the Home Point is set when Phantom has 6+ GPS during the first throttle up (taking off).  If I miss the time window, i.e., taking off without having 6+ GPS, will the Home Point be set whenever the Phantom gets 6+ GPS the first time during the flight, or simply I would not have any Home Point during the entire flight?

it is set as soon as it gets 6+ and you can see it do the fast green flash to tell you that.  if you never get 6+ then, yes, you'd be flying in atti mode then.