Yes, it would have been a problem.
I tend to agree with the concern that Michael addressed. It's quite common in a situation like this to file on the basis of the proposed law when the written comfort is obtained from the government. In this particular situation, the spun-off company—just to answer your question and to give you a concrete example—was actually the subject of a takeover bid a few years later.
Had the original treatment of the spinoff been taxable, the cost basis for the Canadian shareholder would have been the fair market value at the time of the spinoff. But when the amendment applies, you get quite a different result. Instead, your existing cost basis is prorated. In the spun-off company, your adjusted cost base is lower.
In good faith, the shareholders have generally filed on the basis of having a lower cost base and paid more tax, in effect, on the subsequent sale—all in reliance on the amendment. If what had happened was that the amendment had later been yanked and pulled, then they would have been in a position of having overpaid their tax, and it might have been difficult, depending on when that happened, to have a remedy.
So when people plan based on an amendment and do their filings accordingly, and if, generally later, you yank that amendment away, it can create a bit of an administrative nightmare.