The tests to determine residency status are different for student and nonstudent F1 and J1 visa.
As a student you are exempt for 5 calendar years. This is a lifetime exemption. So once you used up the 5 years, you cannot be exempt again on a F1 or J1 student visa.
On a J1 nonstudent visa (like for a postdoc), the rule is that you are exempt if you were not exempt for 2 or more years during the last 6 calendar years (the time counted for this rule would include times exempt on F1 and J1 student visa).
So on J1 you can become exempt again after already being a resident alien if enough years passed.
So on F1 you were exempt from the Substantial Presence Test for 2011-2015. for 2016 and 2017 you were not exempt, same is true for any time you were still on F1 in 2018. When you switch to J1 in August, you will also not be exempt, so a resident alien for the entire year. If you are still on J1 in 2019, you will still be a resident alien, since you were exempt for 2 out of the prior 6 years, same for 2020. However, if you stay longer on J1, you will become exempt again in 2021.
