So why did those Democrats change their votes? Did the Democrats decide they didn't want to be responsible for this bill's defeat?
Some of the Democrats either believed this was a good bill, or they cynically thought that whatever the merits of the bill, voting against AARP was a very bad political idea. Either way, there was a core of Democrats who did not want to obey the order (and threat) from Nancy Pelosi to follow the party line.
It was that change by Democrats, which lead to the final result.
John / Billybob