A change of basis of a matrix is
$$ M'=PMP^{-1} $$
But a vector is
$$ v'=Bv $$
A multivector $u$ can be represented as a matrix, but it is also a vector.
So...
$$ u'=Vu $$
or
$$ u'=VuV^{-1} $$
Is $V$ an arbitrary multivector (provided it has an inverse - thus a general linear group multi-vector...)?