I have an EE3 site, and I want to import data into it from my EE2 site. I'm planning on creating an XML file from the EE3 site that will have the entry IDs and titles, then adding the entry content into that XML file, then importing that XML file into EE3. I know how to construct XML files. What I'm looking for is: is there a way for me to map the XML onto the EE3 entry fields including custom fields; and, can the existing entries be updated if I use the entry IDs as the unique key? I have looked at DataGrab, but I'm not sure about the "updating existing entries" question.
Simply put, if you value your time even a little bit, I would use DataGrab rather than rolling your own import / export routines. Datagrab is simple and it works.
The 'updating existing entries' option in datagrab works OK - but is not perfect: it depends on your data quality and which field you set as the reference etc. As ever with these sorts of things, if you can set up a test system and practice the import offline before you do it for real, probably a good idea.
If you have specific questions about getting it to work, I would put a question into the datagrab support forum.