That is great to hear, but in the documentation it says:
If your osTicket version is older than 1.6, you need to look into migration instead.
And below the heading "Migration" at the bottom of the same page it says:
We plan on supporting data migration from previous discontinued versions of osTicket as well as 3rd party help desks in the near future. Please check the forums for up to date discussions on the subject.
My conclusion is, that there is no documented way of upgrading from 1.15.x to 1.17.x at the moment. Please proof me wrong and direct me to the corresponding docs. Any help appreciated!