is this going into an existing file system or a brand new one ?

It's going into an existing file system unfortunately

bummer, i was thinking of "tape silvering" approach. Not sure you have that many options, 100mbit pipe and 3TB of data will take at least ~3 days to copy. You can adjust throttles if you are concern with clogging that 100mbit link.

There is no way to prioritize individual files – Replicator works on a block level

Whatever blocks are different between the two internal checkpoints that Replicator takes will get transferred.

You might want to temp. raise your RTO

I was kind of figuring 3-6 days to copy it all over.  My concern is that whilst it is copying all this data over the rest of the changes are at the back of this list is that the case?

Can I put this chunk of data on a low priority copy so other changes take priority?

Sorry new to this what does raise the RTO do?

maybe Rainer is thinking about RPO, how much data you can stand to lose if primary copy become unavailable. In your case you have no RPO on the new data while it's being copied, because it's block copy ..who knows what blocks got copied. Once the complete copy is done you can tweak RPO by adjusting max_time_out_of_sync value.