Assuming the regulations are lawful, they contain specific rules for determining the effect of your method change. The rules depend on when the stock was acquired, what kind of stock it is, where the stock is held, whether you had previously sold any of the stock, and a variety of other factors.
One thing to understand is that there are two separate concepts here:
- the method for determining the basis for the securities; and
- the method for determining the gain or loss on sale of the security.
Even if you successfully change (2), your change might be ineffective to change (1) depending on a variety of complicated rules. I don't have time today to write an essay on this topic. Maybe there is an article on the internet that already summarises this information accurately, although I haven't seen one.