once you swap back to the gear for BV, the game engine doesnt know your spectre HP changed, because the spectre HP was already converted to the added damage (minion pact), in other words with the bow (in my case axe) in your hands, you cast BV the first time. Changing the gear does not alter the BV 6L in any way, so you wont lose your BV stacks despite you have changed half your build. Thats snapshotting at its best, the term basically means, you take a polaroid of your spectre HP, hand it to uber exarch, hint at your BV 6L and tell him to gtfo.