Before going to upgrade migrate the data to new tablespace model.
Its must to migrate OATM before R12.2 upgrade otherwise we may face issues while adop enablement.
And OATM utilit is not supported for use after online patching enabled so you must migrate before upgrade.
Please go through the below notes for more information
Oracle® E-Business Suite Upgrade Guide Release 12.0 and 12.1 to 12.2 Part No. E73540-03
How To Run OATM Migration Utility (Doc ID 404954.1)
FAQ - OATM - Oracle Applications Tablespace Migration (Doc ID 418225.1)
According to below link >>https://blogs.oracle.com/stevenchan/last-stop-for-oracle-applications-tablespace-migration
Can I use OATM with EBS 12.2?
The OATM Tablespace Migration Utility cannot be used after you enable Online Patching in EBS 12.2. If you wish to migrate to OATM, you must do so before you enable Online Patching.
If you do not migrate to OATM, you must continue to manage your tablespaces separately after enabling Online Patching.
So according to this still we do have option of going to EBS 12.2 with out migrating existing objects to OATM.
I have gone through the blog
Yes you can use the old structure if you wish to continue the same with R12.2 , if you wish to migrate to OATM you have to do before ADOP enablement .
My suggestion is ,perform OATM before R12.2 upgrade. OATM will increase the performance.