I did my PhD studies in Sweden. In Sweden it is common to write two theses: a licentiate thesis after roughly 2 years, and a PhD thesis at the end. The second one is a natural extension of the first one, so this question came up. One person said one should rephrase everything, but we thought this was silly. Instead, I wrote a preface in my PhD thesis with the following:
This text can be considered a continuation of the so-called
licentiate thesis (Holl, 2011).
In the Swedish academic system, the licentiate thesis is a thesis very
similar in structure to the PhD thesis, that PhD candidates are encouraged
to write approximately halfway through their PhD studies.
A lot of the material published here was already published in the
Self-plagiarism occurs if you claim information is new, when you've already used it before. Therefore, if you write something like this in a preface, you are not committing self-plagiarism.
However, you should still check if your university has any rules against it.