Fair is not always equal. The most important idea to keep in mind is that the students are assessed equally in terms of their ability level.

I think that this is the perfect chance to utilize tiered assessments. Like another user stated, as long as the same standard is being assessed, you have the freedom to choose the assessment tool that you believe is appropriate based on the needs of your students.

It is fair to give John an easier assessment and a harder assessment for Anna as long as the standard being taught is addressed in each assessment. It is important to differentiate as needed for students to test their level of achievement on concepts and lessons taught within the classroom. If John is assessed similar to Anna, Johns results will not be accurate because the assessment was to difficult for him. The same goes for Anna, if Anna is assessed similar to John, Anna's results will not be accurate because the assessment was to easy for her.

With differentiated instruction, it is important to also differentiate instruction. The content that you are teaching is likely differentiated based on ability, so it's fine to give different assessments. I wouldn't say it's necessarily defined as giving an easier and harder one, but it's important to challenge the students appropriately.

I have always differentiated assessments at the beginning of the year. As the year moves along, I slowly make the assessment a bit more challenging. My reasoning behind this is that the standardized test are the same for all students so I kind of feel as if it maybe a disservice to the student if I continue to make it easier for him/her.