A New ABAP Debugger was introduced in Release 6.40. Since that there was a change in its name. Thus the “NEW” name became obsolete. The New Debugger is called just Standard ABAP Debugger. Similarly, the Old ABAP Debugger is called Classic Debugger.
Normally when you start to use the debugger system jumps to the Standard ABAP Debugger right away. There is no possibility to set the Classic Debugger as default debugger in ABAP Workbench settings (t-code SE80 or SE38 -> Utilities -> Settings) anymore. Thus by default the debugger starts as a Standard one. However, if there is a necessity to use the old one it is still possible.
Once you enter the Standard Debugger via following value that is entered into t-code field (so called “ok-code”) the debugger switches to Classic one:
/hset debugger=classic
Similarly to switch to Standard one:
/hset debugger=standard
Switching the default debugger using above ok-codes is valid for the current logon session only. After the re-logging to SAP GUI it won’t be valid anymore.