Stat bonuses on merging a +N unit into a +0 unit?

I know that at +10 you get a flat +4 to all stats, but that very likely won't happen for me since I'm a f2p player, so...

If I merge into a +SPD nature unit (and get the +HP, +spd from +1 merge), and then merge the +SPD nature unit into, say, a +ATK nature unit... for the +1 stat bonus, will it recalculate it as +HP/+atk? Or will it keep the +HP/+spd?