Change flights on day of travel – am I missing something?

So economy plus fares on BA allow you to change to a different flight on the same day free of charge, as I understand it this is with both fare difference and fees waived.

Currently I’m looking at booking a one way from LHR – AMS. For the date I want to travel the time that’s most convenient for me is c. £150 for economy basic, however there is another flight on the same day with economy plus fares for only £80 (which includes free same day flight changes).

In the above scenario, what’s stopping me from booking an economy plus fare for the cheaper flight time and switching onto the more expensive time for free of charge, and saving myself £70? Am I missing something here?