Is it possible to run a macro each time the value changes
in a cell?

If the value change is by user input, then yes. Each worksheet
has a Worksheet_Change event procedure. Put your code there
or call the macro from there, e.g.:

Sub Worksheet_Change(Target As Range)
If Target.Address = "$A$1" then Macro1
End Sub

If the value change is by a calculation, then it isn't easy. Find a
workaround or repost.

HTH,
Merjet