Lathe and Mill are separate program in the OneCNC line. So you could purchase Lathe separately without updating the Mill software.
But did you know that Mill 2000 was updated to Mill 2003 as a free update?
You should call Onecnc and get that update.
Also OneCNC has had two major upgrades since Mill 2003. First there was XP series which had lots of user enhancements ..and Now the current version is XR series. It also has tons of enhancements over XP series.
I started with Mill2000 just like you and you really owe it to yourself to see the XR series..like night and day..no other way to put it.
Also XR Lathe is currently in the final stages of Beta..and soon to be released.