If the declaration you made was true at the time you signed it, then you have nothing to worry about. Anyone can subsequently discover they need glasses and this alone does not have to be declared to DVLA. Incidentally, information on whether you need glasses is found on the back of your photocard licence as a code "01" next to each category. If there is no "01", then it means you declared you didn't need glasses.
If you find that your eyesight has deteriorated so badly that you cannot meet the eyesight standard even with glasses / contacts, then you must stop driving and notify DVLA.
On your first driving lesson (and at regular intervals) your instructor should check your eyesight and note whether you need glasses. That way, if you turn up for one lesson without them, he can give you a helpful reminder before you drive, so that you don't break the law.