The schedule, or the basic plan you see in the standard views, shows
historical fact for things that have taken place and forecasts for things
that have not taken place. If you have two tasks A and B that are each
expected to require 40 hours of work, you have done A and finished it in 30
hours instead, the plan will indeed change the Work field to reflect the
historical fact that the task took 30 hours instead of 40 when you enter the
Actual Work. How could it not show that since that's what really and truly
took place and becomes the basis for scheduling when B should now be
started. IF you still need to know that A was originally thought to be
going to require 40 hours, that's what the baseline is for. You save a
baseline before starting to post progress and it preserves your original
plan for future reference and it will not change when actuals are posted to
the plan.