Relevant rules excerpt:
Psychic Access (Su)
At 4th level, the mindblade gains access to an expanded spell list. She adds two spells from the psychic class spell list to her magus spell list as magus spells of the same spell level. These must be spells of levels the mindblade is able to cast. At 7th, 11th, 14th, and 19th levels, she adds two more psychic class spells to her spell list, following the same restrictions.
When a mindblade takes the spell blending arcana, she gains spells from the psychic class spell list instead of the wizard spell list.
This ability replaces spell recall, knowledge pool, improved spell recall, greater spell combat, and greater spell access.
A level 4 Mindblade Magus uses Psychic Access to add a base form of an undercast spell, say, Mind Thrust I, to their spell list, and subsequently learn it as a 1st level spell known. At level 7, they add Mind Thrust III to their spell list, and pick it up with one of their new third level spells known, and replace MT1 accordingly.
Can they undercast Mind Thrust as Mind Thrust II, even though they never added it to their spell list?
Additionally, can they use retraining to replace having added Mind Thrust I with the level 4 iteration of Psychic Access, getting a different psychic spell that they could have cast at level 4 on their spell list instead?
Yes, undercasting is not dependent on having known the lower level version of the spell. You could just have learnt the highest level version at the appropriate point and still undercast it at every available level below that.
The only Magus class feature available for retraining is the Magus Arcana.
I believe the mindblade can Undercast Mind Thrust spells because:
A) From the Mindblade archetype:
Spells: A mindblade casts spells from the magus spell list as psychic spells.B) From the description of Undercasting
Some psychic spells can be undercast.
Also retraining should not be necessary because the mindblade is a spontaneous caster and:
Whenever a spontaneous spellcaster adds a spell to his list of spells known that can be undercast, he can immediately learn a spell in place of each lower-level version of that spell he knows. In essence, he loses each earlier version and can replace it with another spell of the same level that is on his spell list.
Also retraining should not be necessary because the mindblade is a spontaneous caster and:
Whenever a spontaneous spellcaster adds a spell to his list of spells known that can be undercast, he can immediately learn a spell in place of each lower-level version of that spell he knows. In essence, he loses each earlier version and can replace it with another spell of the same level that is on his spell list.
You would still need retraining to ADD a different spell to your spell list.
Add a spell to a spell list and adding a spell to your list of spells known is not the same thing.
Also retraining should not be necessary because the mindblade is a spontaneous caster and:
Whenever a spontaneous spellcaster adds a spell to his list of spells known that can be undercast, he can immediately learn a spell in place of each lower-level version of that spell he knows. In essence, he loses each earlier version and can replace it with another spell of the same level that is on his spell list.
The problem with this is that Psychic Access only adds spells to your spell list, not spells known. You have to select them as spells known as usual.
While the rules for psychic magic allow you to replace the spell known with a magus spell (or another psychic spell you added previously with Psychic Access), it does nothing for the spell you added to your list via the class feature.
Yes, undercasting is not dependent on having known the lower level version of the spell. You could just have learnt the highest level version at the appropriate point and still undercast it at every available level below that.
The only Magus class feature available for retraining is the Magus Arcana.
The retraining rules do permit you to retrain the entirety of the archetype away, and then choose to retrain back into it, but make different decisions.
I was merely curious if there was a shorter way.
It's the spell itself that says it can be undercast, so you can just add the highest one yo your list and still be able to access the lower ones