This is my very short take on it.
Instead of a Foundation v Higher tier dichotomy, Maths departments and students must henceforth think in terms of a trichotomy or three-way split of 'Standard' (= Foundation), 'Higher' and 'Highest'.
Differences between the tiers are defined, just as they were before, by adding on of topics. Topics indicated for the new Highest tier comprise those previously classed as A* - with a few added. This tier is intended to serve better those students likely to go on to A Level Maths.
The old divisions of Maths, previously four - 'Number', 'Algebra', 'Geometry', 'Data' - have been revised. The new divisions are six: 'Number', 'Algebra', 'Ratio and proportion and rates of change', 'Geometry and measure', 'Probability' and 'Statistics'.
'Ratio and proportion etc' was previously subsumed under 'Number' - and 'Probability' under 'Data'. So these are not really additions - the aim is to give them more prominence.
For detail on this, the source document of authority to refer to is: "Mathematics GCSE subject content and assessment objectives" published online by the Department for Education. This comprehensively lists the topics, using underlining and bold to distinguish Higher topics from Highest.