Technically, this isn’t a macro, but its very easy to set the default printer using the shell command lpoptions.
will list out all your configured printers with their names.
lpoptions -d PRINTERNAME
will set the default printer.
As usual, I found this while researching a support query, and I found it by the standard technique of doing a web search for:
AppleScript set the default printer
Note that the result is not an AppleScript at all, but AppleScript works as a great filter because it means the result is Mac-specific, and since AppleScript can execute shell scripts, it finds any shell commands like this as well.