Please look at Rob's documentation on Incremental Load. Here is the link :
If the data only changes for most recent month then you should build a staging model to keep staging data seperate from the final data. I mean you should keep your previous month's data seperate from the main QVD file and concatenate the data only once you have final data (where no changes required). This way you are still on optimized load and you are also dealing with smaller data set in staging QVD file.
I hope this makes sense.