Change margin only for bibliography pages

You can use geometry's newgeometry (and restoregeometry).

Declare the normal page layout in the preamble and then set the layout for the bibliography with newgeometry. Since a new page layout only really works on a new page we issue cleardoublepage before we change the margins.

documentclass[british]{article}
usepackage[T1]{fontenc}
usepackage{babel}
usepackage{csquotes}

usepackage[left=3.5cm, right=2.5cm, top=4cm, bottom=4cm]{geometry}

usepackage[backend=biber, style=authoryear]{biblatex}
addbibresource{biblatex-examples.bib}

usepackage{lipsum}

begin{document}
lipsum

nocite{*}

cleardoublepage
newgeometry{left=2cm, right=2cm, top=4cm, bottom=4cm}
printbibliography

cleardoublepage
restoregeometry
% not really needed if there is nothing after the bibliography
end{document}