Each version of Access uses a different binary for the compiled version of
the code. They are backwards compatible (e.g. Access 2003 can read an Access
2002/3 format file compiled into an mde on Access 2002, but not the other
way around.) So, when you have been editing the MDB on 2003, and then open
it in 2000, Access is supposed to silently recompile in the 2000 code, and
everything should run sweetly. In practice that does not happen reliably.

We have found it necessary to explicitly decompile whenever you switch back
to an older version. To decompile, make a backup copy, and enter something
like this at the command prompt while Access is not running. It is all one
line, and include the quotes:
   "c:\Program Files\Microsoft office\office\msaccess.exe" /decompile
       "c:\MyPath\MyDatabase.mdb"
Then restart Access (without running any startup code, e.g. hold down the
Shift key), and compact (again holding down Shift.) Then open in the old
version.

It may also help to be explicit about the libaray you want when you declare
references that could be another library, e.g.:
   Dim db As DAO.Database
   Dim rs As DAO.Recordset
   Dim fld As DAO.Field

And the other crucial aspect is to make sure all versions have at least SP8
for JET 4:
   http://support.microsoft.com/kb/239114

That makes no sense to me at all, as DAO is not necessary to use
CurrentDB, as it is available both through DAO and as a property of
the Application object.