Shorter way to convert polynomial in x^3 to x?

[Richard Heylen <richardheylen@gmail.com>]

? f=factor(elldivpol(ellinit([0,1]),13))[1,1]
%32 = 13*x^12 + 52*x^9 + 1536*x^6 + 1024*x^3 + 256
? Pol(vector(poldegree(f)/3+1,i,polcoeff(f,poldegree(f)-3*i+3)))
%33 = 13*x^4 + 52*x^3 + 1536*x^2 + 1024*x + 256


I tried using subst(f,x,y^(1/3)) but to no avail. Is there a short way
of doing this?

Richard