Originally Posted by
Rongway
This change would not affect the swap time after Lv40 (acquisition of M&MII trait, allowing a third elemental stack). From then on, three stacks grant opposite spells a 50% cast time reduction but 0% recast reduction. As Fire III has a cast time of 3.5s, its cast time under the effect of Astral Fire III is 1.75s, but the GCD remains 2.5s. Reducing its cast time to Fire's 3.0s cast would reduce its cast time under the effect of Astral Fire III to 1.5s, still with a 2.5s GCD. Ergo, no change.
A note about the potency you listed for Firestarter Fire III, though. The potency is currently 240. Changing Fire III to be a direct upgrade and replacement of Fire I would make it much harder to cast the "wrong" spell, in regards to when Firestarter procs. Currently, you have a fraction of a second to notice the marching ants and decide to cast Firestarter Fire III instead of Fire I. If you're not quick enough, you'll keep your finger on Fire I and potentially waste and overwrite the proc. If Fire III became the spam spell replacing Fire I outright, this source of player error would be eliminated, and the potency of Firestarter Fire III should be reduced slightly to compensate.