Easy way to switch scylla cluster to large aws instances
You can add a logical DC of the new nodes, run repair and then get rid of the original DC
Add a new DC with the desired instance type (see the procedure @TzachLivyatan posted in his comment)
Wait for streaming to the new DC to complete
Run a full cluster repair -> wait for it to complete
Decommission the "original" DC: https://docs.scylladb.com/operating-scylla/procedures/cluster-management/decommissioning_data_center/