如何在plone中按需更新单个原型对象的模式？

I have hundreds of thousands of objects based on plone archetypes (plone 2.5.X) that need their archetypes schema updated to the latest. The archetype schema migration tool is great for a small/medium number objects but is bringing my server to its knees trying to migrate them all, to the point where I always end up killign the script. I would like to be able to update the schema of one object at a time, potentially as the object as retrieved - is that possible? If not, any other approaches to updating archetype schemas in large plone sites?I have hundreds of thousands of objects based o