Typically it's done like this:
Load new data into staging
Add a new partition to main
Switch new partition in main and staging (now staging is empty)
Switch partition containing old data with staging
Merge empty partition in main table
Truncate staging.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability