Consider an action of a smooth linear algebraic group $G$ on a variety $X$ over an arbitrary field $k$, and the quotient stack $[X/G]$. Let $p$ be a $k$-point of $X$. If the action is transitive (i.e. $G(\bar{k})$ acts transitively on $X(\bar{k})$) then we have an isomorphism $[X/G]\cong BG\_p$, where $G\_p$ is the scheme-theoretic stabilizer of $p$, given as follows. In one direction, send a $G$-torsor $P$ with $G$-equivariant map $\pi: P\to X$ to $\pi^{-1}(p)$ and on the other direction send a $G\_p$-torsor $Q$ to $Q\times^{G\_p}G$.


This cannot be right, because if $q$ is another $k$-point of $X$ then this implies $G\_p\cong G\_q$, but I think we only have $(G\_p)\_{\bar{k}}\cong (G\_q)\_{\bar{k}}$ (it can happen that $p$ and $q$ are conjugate over $\bar{k}$, but not over $k$). If $[X/G]\cong BG\_p$ is wrong, how do I correct it?



Reduction of structure group for stacks
$BG\_p$ and $BG\_q$ are isomorphic if $G\_p$ and $G\_q$ are the stabilizers of points $p$ and $q$ in a homogeneous space. Indeed, the set of elements of $G$ that map $p$ to $q$ is a $G\_p$-torsor, and $G\_q$ is the inner twist of $G\_p$ by this torsor. 


$BG\_p$ is isomorphic to $BG\_q$ because given any $G\_p$-torsor, the space of $G\_p$-equivariant isomorphisms between it and the set of elements of $G$ that map $p$ to $q$ is a $G\_q$-torsor, and this map can be inverted by doing the same construction in reverse. 


