What an old warship needs before receiving missiles

Could 1980s missile launchers be fitted in spare deck space behind an older warship's first funnel?

Yes in principle, but the deck must be strengthened and the added topside weight balanced so the refit remains structurally and dynamically safe.

Open deck area does not by itself make an older warship ready to accept missile launchers. The supporting structure may need reinforcement to carry the concentrated weight. The launcher and missiles also add topside mass, so naval architects must recalculate stability and may need ballast lower in the hull. An apparently simple equipment swap is therefore governed by weight, structure, and stability.