The old rankings would become irrelevant with SY change. Teams will change because most teams will be a combination of two birth year.If you want numbers to be useful they need to be "correct".
A 6 month difference in the age between 2 different groups of players to me needs to be accounted for to make the data presented valuable.
If this type of thing isn't accounted for in the way data is presented large numbers of users will discount the rankings app as biased and unusable.
If you notice I'm only talking about ranking of teams against other teams. Individual team against team predictive outcomes would not need a power addition because you want an analysis of the actual result not what would happen if both teams had players the same age.
After thinking about this I've talked myself into presenting the data as BY-G2010 and SY-G2010 is the way to go. Although it would be nice to have all teams in a single G2010 group it's just easier to keep everything as and not mess around with adding a power result.
8/1/2013-->7/31/2012
8/1/2012--->7/31/2011
8/1/2011--->7/31/2010.
Etc.
Practically every team will change. Some will become better others will get worse. Regardless every team will have be reranked.
Youth soccer rankings don't really matter.