How to "merge" syncs?

I had a tablet, and it was never synced to its Google account. I broke it, and I bought a smartphone, on which I activated the sync on the same account. In the meanwhile I got the tablet repaired, and now I would retrieve all the old data. The question is: if I sync the tablet, will the data on the tablet (WhatsApp conversations, calendar events, contacts, etc) be deleted or merged? And if they will be deleted, how can I merge the old data to my new smartphone?