Expected Goals is basically:
(Expected Shooting Percentage) * (Expected Shots On Goal)
Expected Shooting Percentage is (Average Shooting Percentage Since Lockout) * (% Difference Between Shooting Percentage Of Old Team and New Team in 2008-09 Season)
Expected Shots on Goal is (Average SOG/Game Since Lockout) * (% Difference Between Shots On Goal of Old Team and New Team in 2008-09 Season) * (% Increase/Decrease Expected As A Result of Aging)
There's lots of adjustments that can and should be made here, but they are quite difficult - they include time on ice adjustments, power play time on ice adjustments, power play strength adjustments, penalty killing time on ice adjustments, strength of expected linemates, and so forth; not adjusting for these things likely skews things between 1-3 goals either way (but could vary things as much as 5-10 goals in extreme cases). Regardless, it's a reasonable ballpark metric, and is certainly more instructional than looking simply at the past.