Yes. It can stop working anytime. Previously even in my account, the unlimited data showed an expiry of 5th October. But after few days the expiry date was changed to 13th February (my main plan expiry). So they are again 'gradually' fixing this for everyone it seems.
In their FAQs, they have mentioned that existing customers will get the complimentary speed and data benefits "for the validity of their existing plan". So they may stop this benefit when our plan expires. It would be a welcome surprise if they continue this benefit for a month. It would be as if the old customers are also getting a "free trial". My current plan expires tomorrow afternoon. Will come to know then.