In legacy he did it manually before even Hawke killed him and we can see grey warden in background getting possessed if that was automatic he would transfer after his death and end in Bethany or Carver body. Corypheus could have use his ability to imitate calling even unconsciously so there wasn't much philosophy to it , if Architect had such ability he would quickly figure it out as it would come naturally.
You are forgetting that by the point of Hawke landing the killing blow, Corypheus was pretty much dead already.
Even darkspawn Magisters or Archdemons cannot be hit with that many crossbow bolts, arrows, stab wounds and magic and expect to survive. When we see his eyes briefly go dark, that was his body instinctively seeking out another viable host for his consciousness to jump into, so all Hawke was left to land the killing blow to was an empty vessel.
The most likely reason Corypheus ended up in Janeka/Larius is because they were the most heavily tainted individuals in proximity.
Larius had been a Warden for decades, undergone his Calling and become a ghoul, while Janeka is stated as being a Senior Warden, which based on the WA holding that rank in Inquisition seems to require at least a decade (or more) experience. In comparison, Anders, Carver and Bethany all became Wardens as most within the last six or seven years, so the corruption isn't nearly as strong within them.
The only time we ever see Corypheus or the Archdemons use the ability however is when they are dead or dying, which would suggest it's an entirely instinctual response and not something they have any control over. If they did, you'd suspect that the Archdemon (that seems to have some degree of intelligence) would purposefully try to avoid jumping into Warden hosts altogether.
As for why Corypheus can jump into Wardens when Archdemons can't, I've speculated elsewhere that the soul idea is bogus and it's actually due to the Grey Wardens using a drop of Archdemon blood in the Joining. Because the Archdemon is attempting to possess an someone already seeming to contain an Archdemon inside it, the result is mutual annihilation because the vessel cannot handle it and dies. Without any Archdemon demon blood to present such a problem, Corypheus and the other Magisters are not opposed by the same restriction and thus can take over Grey Warden hosts with impunity.
The only problem with this theory is how does the Archdemon-infused blood of Wardens does not lead to similar problems for Corypheus in their bodies? The only solution I can think of is that Corypheus altering the body of his hosts also changes their blood, something that does happen when Archdemons take over regular darkspawn.
(Course, this is mostly just speculation based on bits and pieces of lore we know, so feel free to take some of it with a grain of salt)