What is meant by the change in residency? I am unclear. I was non-resident in 2017 and the five years before. But resident in 2018 and this despite the long vacation?

This question only refers to changes that happened within the tax year. If you are doing your income tax for 2019, and you were a resident for all of 2019, then there was no change.

If you are unsure of your residency status, you will need to contact the Canada Revenue Agency (CRA) and confirm: https://turbotax.community.intuit.ca/community/federal-taxes/help/how-do-i-contact-the-canada-revenue-agency-cra/00/873448