With ONTAP 8.1, you add disks to grow the aggregate over 16TB and it will automatically convert in the background (no downtime) from 32 to 64 bit. Growing over 16TB is the only supported method. If you search this community you will see some posts of a diag mode in place method without adding disks but that is not supported (diag mode command).
You can also replicate from 32 to 64 with ONTAP 8.1 and have a quick downtime while you cutover to the new volume but if you are on 8.1 then the best solution is to add disks and convert the existing aggr for now downtime.