Also, you might consider Copying approach with SMTape functionality, which can backup your volumes to tape. In this kind of scenario, you can copy data to tape, then destroy aggregates on FAS8040 to release disks from DS4486, detach DS4486 with disks, re-cable DS with empty disks it to FAS8060, create new aggregates and new empty FlexVols, and finally restore data from tape to that volumes.
Alternatively, you can combine my answer with Alex's answer just in case if things go wrong.
Was this from one HA pair within a cluster to another HA pair in the same cluster? I can see that would probably be pretty clean, since the volume UUIDs would remap them into the same SVM they were in previously.
In the scenario where they are moved between clusters, there is an additional need to rehost volumes to a new SVM, re-create export policies, access controls, etc., and that creates risks of allowing access where it shouldn't, loss of ACLs and probably many other things.
Unfortunately this is not part of core ONTAP functionality. We are tracking this as a request for future functionality enhacement in RFE BURT 814304
It is not impossible to do it with current tools, but it should only be done by NetApp Professional Services and it is a very involved task which risks data integrity - for NetApp Staff, please refer to case 2005538876 for details.
Please contact your local account team to engage them