We've been adding such improvements for as long as missiles have existed. Better nosecones, rearrange the interior furniture, new guidance software, etc. When the Cold War was still on, we made two-three foot shots (0) from Vandyland (1) per year. Each one was mined for research and used for tweaking. The two big changes were solid fuel boosters and multiple warheads. Everything else has been minor in comparison.

0 https://www.popularmechanics.com/military/amp9936/behind-the...

1 https://en.m.wikipedia.org/wiki/Vandenberg_Air_Force_Base