Hello Iswari,
As per my opinion, the disadvantages seems to be more compared to benefits.
Detecting/Fixing the issues once you are back to original set-up would be more time consuming.
From Product Master perspective, running BDLS transaction in CRM to change the LOGSYS may not be sufficeint.
For downloaded materials, there will be mapping table in ECC: NDBSMATG16,which strores ID to GUID mapping.
These mapping would not be present in ECC 380. And later if you try to create materials with same material IDs as that of ECC 390, then it may be perceived as duplicates in CRM and material creatin might fail.
Can't you use any other alternatives like replication via IDOC or LSMW, etc from ECC 390 to CRM 480?
You can refer SAP Notes:418886 - Download: Products not found or not changeable
Best Regards,
Shanthala.