CSEs had different syllabuses than O Level, so movement was not easy as a child got older back then - that was one of the aguments for GCSEs (I say my O levels just as they were talking about changing to GCSE).
Er no - very similar, CSE had more practicals but many people did GCE and CSE in the same subjects, if you were borderline C/D it happened a lot.
One thing everyone seems to forget that in the good/bad old days it didn't matter what % you got, the exams were put into catagories, so the top scoring 10% got an A grade, the next 20% a B etc. So if you were in a particularly bright/hard working cohort you had to beat your peers to get good grades.
On the other hand if you were in a poor cohort you had fewer people to beat.
Some teachers used this to the advantage of some students by entering them for O Levels in November, knowing that the majority of November entries were resits.
GCSEs are easier to get high grades in becuase:
a) you are not graded acording to everyone else, if you get 96% in an exam you will get an A, not a B because 10% of the cohort got 97% or above.
b) Everyone does exams now, you never hear of a child who is taking no GCSEs unless they have severe SEN. So yes GCSEs have been dumbed down compared to O Levels because they were never meant to be the same, they were meant to replace GCE O Levels and CSEs.