The MCT for T4-T4S is 55 mins, so the agent was talking nonsense. If you call IB to change your MAD-MEX flight now, they will probably ask for a fare difference, and you will have to pay for your own overnight accommodation, so my advice would be to deal with it on the day. You are protected in case of misconnect, and they can reroute you on other OW partners if the IB flights are all sold out.