Switching theoretical base for Ph.D. studies I have a Master's Degree in a Humanities/Social Sciences discipline. I'm interested in pursuing a Ph.D. studies within this discipline. 

My Master's thesis had a Posthumanist theoretical base. Would it be possible to so to speak "switch lanes" to a methodological-theoretical perspective more aligned with Culture Studies/Sociology? 

I do understand that this all depends on circumstances and University orientation, but would it be possible to give a general answer to this question?
Most of the time, research "resets" between a master's and a PhD, and you are not obligated to continue your research focus between different programs.

(Besides, some faculty change their "base" several times during a career—why wouldn't they let students do the same?)