I think, adding additional transfer partners/increasing the ratio was anyway appreciation, so no prior notice/announcement is necessary.
Also, M&S grabdeals might not be considered that big a devaluation.
When Axis decided to exclude Insurance payment, they did give considerable notice. So, for any devaluation of transfer ratio, I am hoping that they will do a similar thing.
Also, I have seen this with HDFC Infinia that whenever there is any devaluation, there is generally a month or atleast few days notice given.