This isn't allowed because you'd end up comparing apples and pears when analysing data over multiple versions. If the programme specific Indicator measurement unit is wrong, stop using the Indicator anymore and create a new one. Only the Baseline & Target measurement unit can be modified.
The measurement unit is part of the definition of the Indicator. We cannot allow to change that during the lifetime of the Programme because it will corrupt all reported achievements against it.
E.g. your Programme contains an Indicator "Roads constructed" with Measurement Unit = "kilometres" and you already reported "1000" against it in AIR 2015 and now you would change the measurement unit to "metres".