Thanks for pointing out that this doesn't matter when the CG is all within the 0% bracket. Of the years I checked, there's only one (2021) when amending would make a difference.
When a mutual fund is set to cost averaging in Quicken, it doesn't record lot information, so your suggestion isn't feasible. It's impossible to reconstruct the lots even if I wanted to. (I also disagree with the idea, but that's moot.) Cost averaging is standard with mutual funds, so when the brokerage knows the cost basis, it doesn't allocate sales to specific lots: it reports the proceeds, the cost basis, and that's all. It's also all that TT asks me for. So I need to work with what I've got, as above. I also have to work in terms of the TT screens rather than where things end up in forms, which is why I asked my questions the way I did.
