Does adding a partition require reprocessing the entire measure group?

I'm using SSAS 2008 R2.

I need to add some new partitions to update our "Prior Years" historical cube with another year worth of data. If I add a new partition will it require reprocessing the entire measure group (all the partitions) or will it leave the existing partitions an a processed state so all I need to do is process the new partitions to complete the data set?

I ask because the data sources only contain the recent data - if I have to reprocess the entire measure group (all partitions) I will have to rebuild the historical data in the database that feeds it. I would prefer to avoid that, and if I know in advance it will help me decide how to proceed.

Thanks!