Redshift Managed Storage vs Spectrum

If the only purpose of the data to be in S3 is for access via Redshift, they can certainly switch to RA3. There is no special command to load the data. Just use a COPY statement.

However, if the data which has been offloaded to S3 is used by other processes (i.e. EMR, Athena, etc.), it will no longer be accessible by these and you should continue to use Redshift Spectrum.