I would advise DD to speak to her teacher.
I would also look carefully at the raw marks required for each grade / UMS if that's possible.
Sometimes, scoring as little as 6 or so more raw marks can push you up as much as 10 UMS marks. For some subjects however to push up 4 UMS marks you need to score an additional 10 raw marks. So, worth checking.
There are very many variations of scoring bewteen subject and exam board. It's not so simple a decision. And to add, it changes from year to year of course - no crystal balls - but experienced teachers have a good idea of requirements.
That said, if your DD feels she can achieve better without jeopardising her second lot of AS exams then it is worth considering. There are plenty of tales about getting to A2 exams and the end result is a grade for university missed by 2 or 3 UMS marks!
A levels, to a point, is about accumulating as many UMS marks as you can. It ultimately is about the final grade. A2's are harder, marked more strictly etc. My own DD got good A level grades only because she went into the final A2 exams with good UMS marks attained in her AS exams. She had a buffer - and jolly well she did too!
I'm a parent - not a teacher, so offer an opinion based on my experiences as a parent, and do advise you or your DD speaking to her school. It does depend on subject / exam board / raw marks, what other exams she has, and is she likely to get more UMS marks?