My switchover, as I described above, was flawless.  I spoke to Spark to set a disconnection date, and then I gave that date to Orcon, and they switched over without any hiccup.  I just got my final bill from Spark, and they stopped billing for my connection on that final day.  At most, one day of overlapping charges, but I can't imagine it being any more seamless than that.  No interruption of service, and no overbilling.