When selling stocks or funds, the default cost basis that an investment company uses for IRS reporting is FIFO. After selling a bunch of assets, can the investment company actually require you to change FIFO to LIFO within 24-48 hours, or are they required to allow it 'till the end of the tax year?

I assume that I'm not in violation of rule #2 since I'm not asking for advice about my investments or financial planning - rather, about the requirements placed upon trading companies with regard to how they report to the IRS and the window of opportunity for altering how that reporting is done.

Unless it's critical, I'll not say who I'm investing through just yet.

One of the larger investment companies in the US is currently telling me that they cannot change the cost basis for a bunch of sales I did a couple months ago. I sold about half of my total assets in a number of investments. If I have to use a cost basis that is based on the first half of the purchases, I'm going to have a massive tax load. If the IRS is told that the cost basis is based upon the latter half of the purchases, I may actually be showing a loss.

This becomes important because I want to pay the tax on the earliest purchases in a year when my tax load is the lowest, like next year when I'll be retired and have no salary income to report for the year.