You can do this, however, don't expect the movements to happen very quickly.  When you move the SG to the new policy, all the data on the SATA tier will be shown as 'out-of-policy' in the demand report.  Most of the movement performed to get the data off SATA will be performed by the FAST VP compliance algorithm which is not designed to move a lot of data quickly.  If you have the default relocation rate set (5), the compliance algorithm will move 2GB approximately every 10 mintues.  If you want the data moved more quickly, you can use VLUN.  Starting in 5876 and SE 7.4, the symmigrate command has an optional source pool flag.  With this, you can specify the source pool as SATA and the target pool as FC.  Only the data in the SATA pool will be moved as part of the migration - any data that FAST VP promoted to EFD will be left alone.
I have a policy which is 7/100/70 currently and want to eliminate the 70% SATA pool for one of the hosts which is perfoming poorly.  Can I create a new policy 7/100 and add the host to this pool and let FAST move the extends from SATA to FC?  I do not want the extends in EFD to move.
Move extends from SATA to FC in FAST