Do geshe degrees change over time? I have read several different conflicting pieces of information about geshe degrees.  I assumed geshe is a rank given at the end of study.  But, according to Wikipedia:


  At the conclusion [of their studies] the abbot assigns each candidate to a category of geshe according to his ability. There are four such categories, Dorampa, Lingtse, Tsorampa and Lharampa, Lharampa being the highest. After this, in order to qualify, the geshe candidates are not allowed to miss even one of the three daily debate sessions during the subsequent eight months.


Does this mean that one can first get a dorampa and then a lingtse later, similar to how lay people earn a bachelors and then a masters?
The difference between the geshe degrees is comparable to the grade you get on an exam. 

From TibetCulture.net:


  Geshe Lharampa is the highest degree of the Geshe system, equivalent to the doctorate degree in Buddhist studies. The other three lower-level degrees, namely Dorampa, Lingtse and Tsorampa, will also be conferred according to candidates' performance in the dialectical debate.


And:


  Such a challenge as it is, it usually takes decades for a few distinguished lamas to succeed.