It depends whether you want the symbol to change or just the 'Signal Name', and this may depend on what the symbol looks like. If it is just a matter of changing the text then you can simply use Find/Replace from the Edit - Find/Replace - Text command.
Go to the Replace tab:
Expand the 'More' button:
'Look for' 'Properties only'
For an Integrated project:
Select Global Signal Name = DGND and replace with AGND.
For a Netlist project:
Select NETNAME = DGND and replace with AGND
If you want to change the symbol use the Edt - Find/Replace - Replace symbol/part dialog.