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.
How to offload historical data to Redshift or S3?Accepted Answerasked 2 years ago
Athena and Redshift Spectrum performance best practicesAccepted Answerasked 4 years ago
use Redshift Spectrum to query both Redshift table and s3 fileAccepted Answerasked 2 years ago
How is data returned from Spectrum to Redshift cluster?Accepted Answerasked 3 years ago
Redshift Insert into s3 file data into existing tableasked 6 months ago
Is it possible to have a Redshift Spectrum schema access multiple accounts data ?asked 6 months ago
Spectrum on Redshift Shift Serverless with Enhanced VPC routingasked 2 months ago
Redshift Spectrum giving IERR_READ_ERROR (Access Denied) errorasked 3 years ago
Distinguish Redshift Spectrum from Redshift CostsAccepted Answerasked 4 years ago
Redshift Managed Storage vs SpectrumAccepted Answerasked 2 years ago