"To select a mythic spell, you must be able to cast the non-mythic version or have it on your list of spells known." If you don't know it and you can't cast it, then you can't select that spell. Seems straightforward enough.
Most of the classes that can retrain spells known do so slowly and their interaction with Mythic Spells seemed obvious. Until the Advanced Class Guide was released. The Shaman at 6th level or higher can use the Wandering Hex class feature to select Arcane Enlightenment from the Lore spirit, choosing a handful of spells from the sorcerer/wizard list to prepare that day. The next day, they're gone, and the shaman can choose the hex again and select a new loadout.
Issues of "balance" aside, do your Mythic Spell picks get refunded if they're no longer valid choices? If so, do you have to choose them again immediately, or can you wait until you've used a class feature to choose new spells you couldn't normally pick? If not, are they retrainable for a small commitment of gold and time? Or do those Mythic spells just stay there until you pick those spells again and get them back?
Rules-as-written is strongly preferred, as I like to come from a strong understanding of the rules prior to making a GM call to change things or deny a proposed change, but statements of developer intent (even if unofficial) or comparisons to how RAW handles similar situations are also very helpful.