Are files overwritten in place by sftp?

In the event one copies a file or directory over an existing
file or directory is the duplicate written to the same physical
blocks in storage, or are new physical blocks written with the
old blocks marked free?

The situation in mind is using get -r to copy a remote directory,
then changing or adding a few files to the remote directory followed
by using get -r to get the directory a second time. Clearly, changed
files will require writing new blocks on the destination, but what
about files with unchanged paths and contents?

The question is motivated by concerns about limited-life storage
media such as flash.

Thanks for reading,

bob prohaska