Each year you have the grade boundaries are the same UMS so I think they adjust raw marks according to how hard a paper is - so in theory if there is a hard paper one year it doesn't mean that less people get an A grade for example.
I'm not exactly sure about the details of UMS, but my question was about how one can take an AS in a subject with one board & switch to another board for A2, because the grade at A2 is based on the results of A2 & AS modules. So if you didn't take your AS modules with the same board how could you get a grade for A2?
The only way I can see this could work would be if the board you take the A2 modules with can transfer the AS module marks from the first board. The only way I could see this could be possible if the UMS marks are considered to be universal across all exam boards.
Have no idea of he answer though