In-place updates in a MemTable?

[Artur Siekielski]

Hi,

if I issue an update to a row (non-PK keys), and the old row is still 
stored in a MemTable, is the old row updated in-place in the MemTable, 
or a new entry in the MemTable in created and the old row will be 
removed only by the SSTable compaction process?

(Cassandra 2.1)