Changing mergeFactor in mid-stream?

[Barnett, Jeffrey]

The http://wiki.apache.org/lucene-java/ImproveIndexingSpeed page suggests that 
indexing will be sped up by using higher values of mergeFactor, while search 
speed improves with lower values.  I need to create an index using multiple 
batches of documents.  My question is, can I begin building with a high 
mergeFactor for the bulk of the load and then switch to a lower value for the 
final batch?  I build the indices offline, and only swap them to online when 
complete.  The online index is never updated.