If an investor does a tax-free conversion of one share
class of a fund into another share
class, e.g. investor class to admiral class, there are no tax consequences.
But if an investor sells one fund and buys another, even if the second fund is a clone of the first, two taxable things happen:
1) If this is a taxable account, the investor may recognize gain on the sale, and
2) The old fund may have to sell assets to satisfy the redemption, and in the process generate cap gains that may be distributed to the remaining (old) fund shareholders.
That's what happened here. The employer plans were not exchanging share classes in the same fund, but were literally selling shares of one fund and buying shares in a virtually identical but separate fund.
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The simple response to your second observation is yes, that's generally correct. The complete answer is a bit more complicated.
Any fund, whether active or passive, ETF or open ended, can alter its portfolio holdings in a couple of ways:
1. Sell the old holdings, buy new ones. This could generate cap gains if the old holdings have appreciated.
2. (a) When investors want to redeem shares, instead of handing them money, hand them back the old holdings that the fund wants to get rid of. Yes, even OEFs in theory (but rarely in practice) can do redemptions in kind (see link below). For an ETF, the bundle of holdings that the investor receives is called a "redemption basket".
https://www.dodgeandcox.com/redemption_disclosure.asp (D&C redemption baskets)
(b) When investors want to buy shares, rather than using cash the fund (ETF) requires the investor to hand over a bunch of new holdings. This bundle of holdings is called a "creation basket".
Usually the creation basket and the redemption basket are the same, but they don't have to be. This gives the funds the ability to tweak their holdings, handing off old holdings it doesn't want and taking in new holdings that it does want.
I doubt this is how a passive ETF would deal with a full index reconstitution, but I could envision it being used to replace a single security. So just because an ETF wants/needs to change its portfolio, it doesn't have to recognize gain on the old holding(s).
(From iShare IG Corp Bond ETF
LQD's prospectus: "Creation and redemption baskets may differ ...")