I think we may be talking about a couple items here. I would like to explain some terms.
pre-renal is NOT early renal disease.
When talking about pre-renal it is an anatomic reference to azotemia. Just as post-renal causes of azotemia do not refer to complete renal failure. Pre-renal indicates factors that can raise BUN and creatinine before they reach the kidneys. If the BUN and creatinine rise because of the kidney, they are renal causes. If the BUN and creatinine rise due to causes after the kidney (such as an urethral obstruction) then it is called post-renal.
The most common and usual cause of pre-renal azotemia is dehydration.
Also note that you can have pre-renal, renal and post-renal causes of azotemia all together or in combinations. So just because an animal has pre-renal azotemia, it is does not mean that it will become renal or post-renal azotemia.
Hope that helps