Those people tended to be moderate voters -- Specter's people -- and without them he cannot win a primary. But with them staying as Democrats, he could actually start with a leg-up as a Democrat, just in case any liberal challenger might try to take him on in the Dem primary.
And the other side of this coin is that the folks who remain as registered Republicans are now proportionally much more conservative than the state GOP was before.
Remember that Specter only won his 2004 primary against conservative challenger Toomey by a 51%-49% margin -- and that was with the full backing of the Bush White House. So if we just made that demographic adjustment, Pat Toomey would have probably won the 2004 primary with all other issues being the same. And the stimulus is the final nail. The stimulus vote, and the lack of a powerful Republican establishment these days, made a defeat in the primary seemingly inevitable.
A Rasmussen poll from just a few days ago put Toomey ahead by a 51%-30% margin. Specter was viewed unfavorably by 55% of the GOP electorate, compared to only 42% favorable. The pollster's analysis also pointed out that 79% of them had a favorable view of the Tea Parties -- not exactly a receptive audience for a pro-stimulus Senator. This was the first poll since Toomey officially got in, but other polls before that also showed Specter way below 50%, with a high undecided number, and the only question was whether Toomey could pick enough support to pull ahead.
And finally, it's important to remember another aspect of Pennsylvania politics: If he had run in the Republican primary and lost, he would not have been able to pull a Joe Lieberman and run as an independent. They have a "Sore-Loser Law" that forbids that very maneuver. So his choices other than retirement were to run as a Republican and probably lose the primary, run as an independent and face some serious structural disadvantages, or to take a chance on going over to the Democrats. And given those sets of probabilities, switching to the Dems became the obvious choice.