You know, the more and more time passes, the more it seems that the so-called "retcon" powers did not actually "retcon" anything that wasn't explicitly retconned in the comic itself. I have no other way to explain this bullshit.
As with most things Homestuck, I believe that retcon powers can be categorised under "complete fucking horseshit".
With [relatively, at least]new information about the superweaponhousearrowthingy, allow me to attempt to categorise the retcon powers:
The superweaponhousearrowthingy was glowing because it contained Rose's, John's, Dave's and Jade's souls. John absorbed those souls, gaining the retcon power. I therefore theorize that the retcon power is in fact a synthesis of the Beta kids' aspects:
Time: The retcon power allows John to travel through time. Simple enough.
Space: The retcon power allows John to travel through space. Self explanatory.
Breath: The retcon power [A least partially]frees John from adherence to the traditional rules governing the use of time travel. Breath is associated with freedom and inspiration.
Light: This one I'm less sure about. Maybe having him appear and disappear at fortuitous times and places in order to further the alpha timeline? If John hadn't "randomly" disappeared mid-beatdown with Caliborn, he might very well have killed him, breaking his unconditional immortality, which is what defines the alpha timeline. That's sort of within Light's portfolio, isn't it?