I don't lock my client's tax returns, maybe I should and would like to know the pros and cons.

I often go back to prior year tax returns for numerous reasons and am receiving this year a notification that calculations in the prior year's tax return have changed resulting generally in overpayment of tax.  Is there an easy way to amend these tax returns?  It seems that when I click on "View Changes" I go straight to the X and there is no description of what actually changed that resulted in the change.  How am I supposed to amend these tax returns, which I assume should be amended?