You can request an IEE whenever in the process you would like. However, depending on your reasons for the IEE, it may be in your best interest to have the school district complete a reevalution as the data from the initial evalaution will be considered out of date and you may not have as much teeth in a argument that the district's evalaution was inappropriate. Remember that if you request an IEE and the evaluator's results/recomendations align with the district's, you will be responsible for the evaluation costs. The district may have a leg to stand on if it ever went to due process if they dont want to allow the IEE due to the time lapse since the purpose of the IEE is to demonstrate that the district's eval was inappropriate. Due to this, under normal circumstances, I'd have the district reeval then if you still have concerns then request the IEE.