You might want to use Dynamic.
In[1]:= a = 4*4
In[2]:= Dynamic[Sqrt[a]]
If you then change a, it will automatically change the square root as well. You don't even need to re-evaluate. I think that the whole point of the Out[3] labelling is that it gives a unique identifier to every output. If you want content to change dynamically, when you change values in the notebook, the preferred construct seems to by Dynamic.
Cheers,
Marco