Hi, I feel your pain, it's a pretty scary idea. I was exclusive from 2011 to 2015/16.
How many clips do you need to upload? I think I had around 6000 at the time and it was a bit of a slog to copy and paste all the keywords and titles over.
Being non-exclusive has taught me better techniques on how to store and library my clips at home and how to create .csv's for all the sites.
Yes all the sites accept Getty / istock releases. I actually still use the old istock template as I never got around to changing it.
In a nutshell I'm pretty confident that I'm earning a lot more being non-exclusive than being exclusive. I still have my files on Getty/istock and in truth brings roughly the same as my shutterstock sales each month. (I wouldn't delete yours as the amount of time and effort taken to upload them wouldn't seem right). I haven't really seen a drop in my income from Getty but the premium membership sales are driving me nuts.
It is hard work being non-exclusive. I spend half my time uploading and csv(ing) compared to before. I'm ok with that. I do this full time but if you are part-time - it is a lot of work.
There are companies out there that you upload your footage to and they do all the uploading to all the sites. I don't personally do this but you could check out
https://www.blackbox.global/Good luck!