How many active tasks are out there now? Unfortunately, the only think I can think of is to use K2's management site to perform a "Go To Activity" on each process instance with a task to essentially make it regenerate the task and send the e-mail out to the new address. There isn't a good bulk way to do that, though, at least not in the management site.

I was looking more towards a script or stored procedure, not sure if this is even possible. I will need to do around 3*personal details removed* users at a time and as quick as possible. So the management idea would be my very last resort.