Golden Rule! You cannot change the last digit, and if it's 1 or 2, forget it. There are a couple of oddball models that may be outside that rule of thumb, but they are so rare.....
xxxx5, xxxx6 and xxxx7 should reflash within their OWN range, but you cannot flash a 5 to a 6, or a 6 to 7, or any other permutation.
The 5 stops at 48x write, 12x RW
The 6 stops at 52x write, 24x RW
The 7 stops at 52x Write, 32x RW - this is about the limit for CD-R technology.. a few drives go as far as 54x, but it's clear that progress is limited - by rotational speed, and by the double whammy of diminishing returns - as with it already being pretty fast, there's less and less time to be saved.