Incepts are intended to be fixed
Incepts are only described in one section of the Storyteller's Guide, so our information on them comes from such.
Each angel has a number of Incepts equal to its Rank.
Page 83, emphasis mine.
If Incepts could be accessed as needed, the passage might instead say "can choose" or "can employ", specifying when and what conditions would allow Incepts to be "swapped out" (eg, end of scene, if not in use.)
Furthermore, consider Influences:
[Angels] begin with dots in Influence equal to their Rank.
DtD, Page 343
[Angels] begin with the Twilight Form Manifestation and a number of Manifestation Effects from the list below equal to Rank.
DtD, Page 345
Numina follows the same pattern on Page 349; all state that Angels have a discrete set of these powers, implying that they are permanent fixtures on their sheets, instead of dynamically assigned. Because Incepts uses verbiage that implies ownership rather than access ("has" rather than "can use"), it makes sense that Incepts are intended to be fixed.
It is understandable that example Angels found outside of the Storyteller's Guide would not reference this material, but it is unfortunate that no examples within this book offer a concrete example of an Angel's Incepts. The Superintendent would have been an excellent choice, but he is built before the section on Incepts.