An UPSERT SELECT would be the easiest way to copy the data, but you would have to significantly increase the Phoenix query timeout configuration values to accomplish this. It may be easier to export the table contents as CSV/TSV and then use the CSVBulkLoad tool to load the data in the new table.
Remember that salting is essentially adding a leading "bit" of entropy to every row key. This means that every lookup via primary key now actually performs $numSaltBucket lookups whereas it would be a single lookup on a table without salt buckets. This difference in execution is entirely transparent to you as an end-user, but, depending on your queries/usage, you may see some queries taking longer than without salting.