Planet13 - 5:12 pm on May 16, 2011 (gmt 0)
Are we at risk if we stop paying for these old links and have them removed even though they are on bad sites?
How bad are the sties on which they are placed?
It may be that google has discounted any value from them already.
Do your competitor's sites who are ranking in the top spots have links from those sites, too?
As for mitigating the loss, the only way I can think of off the top of my head is to get more links from better quality sites that you DON'T have to pay for.