I would clarify that the NWEA recommends that students receiving 6th grade level math instruction should be assessed on the 6+. (This does not actually apply to the reading test, which has more continuity of skills and content from level to level.) It appears that there is instructionally-relevant material on the 2-5 test up to early seventh grade topics. This suggests that students whose instructional level is late sixth grade or lower will be at least somewhat adequately assessed by the 2-5 instrument, but that students whose instructional level is any part of seventh grade or higher will not. It accommodates those in fifth grade performing up to about a year ahead, but not those beyond that.
The actual change in NWEA recommendations is that schools should use the same level throughout a single school year, rather than their previous guidance, which recommended switching from the lower level to the higher level if the fall score exceeded a certain cut score. This made it quite challenging to monitor any sort of progress over the school year, due to the drop in scores typically observed when moving from a ceiling score on a lower level to a higher level test.
