Looking online, stage 2 is thicker and can be harder to first, and had higher levels is silver other minerals as well as iron. So not the same, and not interchangeable.
The follow on is probably better for getting iron reserves back up though, especially with mix feeding, as breast fed babies levels are often v low by 6 months.
It may have been invented to get round the bansb on advertising, promotions etc, but it also sounds as if it is slightly better for older babies as well.
Given breast milk composition changes as a baby grows, formula milk also changing makes sense to me