Hi. Welcome to the forum.
I'm going to disagree with you. The PHB says this on page 201, "When a spellcaster casts a spell using a slot that is of a higher level than the spell, the spell assumes the higher level for that casting. For instance, if Umara casts magic missile using one of her 2nd-level slots, that magic missile is 2nd level."
As you can see, the spell level can and does change if you up cast. That means that a spell on the Aberrant Mind's spell list that is capable of being cast using a higher level spell slot changes spell levels when the Aberrant Mind chooses to up cast it. A 9th level Aberrant Mind that casts Summon Aberration can opt to use a 5th level slot if one is available. Once the spell level is set at the new level, the Aberrant Mind can choose to expend that slot or use sorcery points in place of the slot using Psionic Sorcery. What the Aberrant Mind cannot do is up cast past his highest available spell slot, since it requires an available slot in order to move up in level.
What also happens is that if you were to say use that 5th level slot to cast Detect Thoughts, which doesn't gain any extra power from using a higher level spell slot, it would still take 5 sorcery points to use Psionic Sorcery, as it still becomes a 5th level spell per the above rule. Seems like that would be a waste of points and a slot unless you HAD to do it for some reason.