āruppa & arūpa

Dear all,

The āruppa in the early texts has clearly been changed in the commentaries to arūpa, as in arūpakammaṭṭhāna and arūpajjhāna. I can accept that rūpa = ruppa. But what's the justification for changing the prefix ā to a?

With the new word, we assume āruppa means "formless", but could it possibly mean something esle?

with metta,
Kumara Bhikkhu, ven.