This is just experience but in around 50% of remarks the grade stays the same. In 25% in may go up if the grade is just short ( by no more than two marks usually) and in some cases, the mark actually goes down, even if it does not change the grade
Unless there is a clear error in marking ( this is why you send for the paper by the fastest method possible immediately after the results come out) the grade is likely to stand. It may go up and still be a C. It may go down and still be a C. I do not think, with the result as it is, the grade will drop to D (but it is within possibility, as with any remark).
In 20 years of pupils asking for remarks , often without my knowing anything about, I have had only ONE student whose grade went up and with him, we knew there was an error ( a glaring one - a question had not been marked) when we sent it. I have known several drop in marks even though the grade has not been affected. I have known fewer go up a mark or two but still remain the same grade.
I had one student who went ahead with a re mark and ended up a grade lower ( her mark was barely above the grade boundary she had received and she went from a pass to a fail ( this was A level in her case).
WJEC particularly, is usually pretty good at being consistent in marking in my experience.
But if you want a re mark no point in waiting , go for it. Most teachers have little or no real say anyway, its a parents and SLT decision.