I have been with giffaff for some years, and in that whole time I thought giffgaff was as unlikely to scam people as I was to change to another provider, which in hindsight shows how naive I am (and how, respectively, it is time to find a newer provider with better principles and transparency).
I think you know where this is going...
...all the customers who religiously pay their £20 each month in exchange for an unlimited data, calls and texts plan would have been stunned (and those who are unaware of the changes and yet to find out - the hard way) to find out, without any warning, their goodbag no longer contains the unlimited data you were expecting.
This bold move, by giffgaff, would have caught people off guard since people, at least I, were not given sufficient and reasonable notification. I received neither a text or nor a diversion explaining the changes. It is wholly unfair to expect that users accustomed to purchasing their goodbag's do so reading the exact details, of the plan every month, expecting changes, especially when it is not a typical one.
This is a really underhanded move and definitely not one I would expect from a well established firm such as giffgaff. In future I would suggest that you, giffgaff, inform your customers, through reasonable means, of such changes. This could either be through a text or a diversion, in the payment page, with a clear, bold warning underlining the relevant changes to the relevant goodbag (or highlighting all the changes altogether).
Take pride in the fact you have (had - we'll see all the unhappy bunnies over the course of the month, believe me) a loyal customer base. Take responsibility.