Honestly, there are so many ways GameFreak could handle the Apricorn thing; there's just no way to tell what's going to be the outcome. I think, oddly enough, Notched-Ear Pichu might play a hand in deciding this, however. Let me explain.
If NEP is anything like Wormadame, Deoxys or Gastrodon, you'll never be able to trade it to DPPt, ever. In that respect, it would be similar to an exclusive item in that it's trading functionality would be limited. Or, to draw a more direct comparason, it's like trying to trade Rotom's Frost forme to DP. That is to say, it's impossible, because the Rotom forms don't exist there. Thus, Rotom has to revert to it's base state.
Assuming NEP cannot become a normal Pichu, it's forever stuck in HG/SS. The only way NEP could revert to a normal Pichu is if GameFreak used one of the unused bits in the Pokemon checksum to denote an alternate sprite, but kept the same index number for the Pokemon itself, so that DPPt just views the bit as nothing and ignors it.
If I'm not mistaken, there are several unused bits in the Pokemon checksum, still. So really, it's still a coin toss.
Will NEP have a seperate set of level-up moves, or will it have a different gender ratio or base stats? If so, then it will have to stay in HG/SS. In any other case, they may have developed a workaround.
In the event that NEP is just a cosmetic alteration of a normal Pichu with some event moves (moves it cannot relearn with Heart Scales), then it also opens up the door for any other "visual quirks" GameFreak wants to implement in any other Pokemon. Say... uh... a Sharpedo with a scar over one of it's eyes, or a Mawile with a striped pattern, etc.
So, likewise, if they used one of those blank checksum bits to denote a range of Pokeball sprites outside the usual range (Heavy Ball, Level Ball, etc.), they could also assign it a normal pokeball bit for... say a regular pokeball. When a Snorlax in a Heavy Ball is located in HG/SS, it would show as being inside a Heavy Ball, as the game would know to override the Pokeball bit for the Heavy Ball bit. However, when traded to DPPt, the normally unused bit would be discarded, and it would show up as a normal Pokeball.
All just theories, mind you.