I have a ton of transactions which I need to update the segmentation on… about 70,000 transaction lines. What are the main risks in doing this? All I intend to change is department + location on the line. Is a map reduce script the best way? Have you done a bulk update like this before?

I'm not an accountant, but as far as I remember locations are something you want to review carefully, locations behave as an accounting center of some sort, and changing the location can potentially affect your inventory costing and/or cogs because some accounting-related data is tracked at the location level for inventory. Again, I may be wrong, this is from the top of my head.

I am an accountant, and @alien4u is exactly right. Those are GL impacting segments, and you can end up in a lot of trouble by changing those for exactly the reasons outlined. Locations = Warehouse. If you change the location on a sale or purchase transaction, then you've changed your warehouse and your inventory levels in that warehouse. If you've got inventory details (lot/serialized inventory) or inventory statuses, or bins, then you've got to deal with all the potential pitfalls that come with those as well. Test it out in Sandbox before you run it in production to make sure it does what you think it should do and doesn't produce any unintended consequences.