There is no setting for 3 digit or 4 digit. It will immedediately dial an extension that matches. If you have an extension 210, then as soon as you finish dialing 210 then it will call that extension. If you had an extesion 2000, but no extension 200, then it will wait until the 4th digit is pressed.
The hard part will be converting the digits to 4 digits in the ame range. If you have an exteion 200, then you can not create a 4 digit extensio starting with 200, such as 2000, 2001, ext. Same with 201 can not have extensions 2010, 2011, ext.
If the extinsting extension number will conflict with the new extension numbers, then I would 1st change them to somethin elso, 300, or 3000, or something starting with 4 or 5.
You may want to check the Dial Number Plan in the System tab to see what temperary numbering won't conflict with something already in the system. You can't use 9 for any eextensions (LCR code), and 8 is usually predefined (but you can remove it if you don't use it)and other numbers may be used for PHNs, or MGPs, etc.