D-Class is typically for life, while it is stated that a D-Class is "executed" at the end of the month there are some theory's that in reality they are just relocated around sites at the month mark, pointing to the fact that executing an entire workforce every month is unsustainable.
There are some very rare and extraordinary cases where a D-Class is "promoted" to an extent, as some of you may know Class-D's that exhibit a rare trait, such as suffering from a rare disease or are disabled may be exempt from monthly termination due to how hard it is to find these people.
There is one case where a Class-D was promoted to a "Class-B", mainly in that this Class-D was an expert hunter and tracker and proved highly useful in the examination of a SCP species. (I forgot its classification).
There is also one case where a Class-D was promoted to a SCP, again I forgot its number.
As far as I know however no Class-D has been admitted into the Staff Ranks.
SCP:CB Administrator. 99% of my statements are my own and do not represent the official developers viewpoint.