Not sure how much data you have copied, how long it took and whether the source data is still available?! Because the easiest would be to perform the object copy with proper options. Source objects may get recycled, but there's also a possibility to keep them protected. And also for the destination objects a protection could be specified. So you could have all you need by just performing the object copy with the right options. Now, if the source data is gone already then this is even not an option anymore I guess.

I also want to give a warning here. When the retention period is changed afterwards through the GUI or with omnidb, please be aware that this is changing the retention for the object in the Internal DataBase. If for one or another reason the object would get imported again in the future then the original protection will be loaded into the IDB again. A kind of related Support Tip I released in the past is this one: (DP) Support Tip: Set protection explicitly for replicated objects.

Wanted to provide you with the hints above already. I'm assuming you may already be further in the process by now? There are probably different ways to handle this. So if you were already scripting something in the meantime, but still have questions or doubts then please share the current status.