Guidance on migrating from Essential to Zephyr

Has anyone had experience in migrating test cases, test cycles and test executions from Zephyr Essential into Zephyr (Scale).

I've reached out to Smartbear support who have just pointed me to the various manuals.

We will use the REST API. Extracting the data from Essential and then importing into Zephyr.

What I am after is any experience in this. What are the traps to avoid? What are the tips to take onboard? I am assuming it will be a multi-step approach. I am keen to know whether historical test execution dates can be preserved. 

I am aware that attachments can't be migrated, and neither can test case audit history, which is not ideal.

thanks

Anthony