SubTypes - New GDB from Old Data

Hi all,
I'm modifying one of our datasets to use SubTypes (which is one change in amongst others). Over the past fortnight, I've been sorting out the schema for the FGDB. However, I've noticed that when I copy the features across, the Default SubType takes precedence (even if the subtype field I'm using exists in the existing dataset).

I've read up and accepted that... But the only way I can think, to get around this, is to have additional fields and once all data is copied across, is to then use the Field Calculator and get the values from the other fields.

Has anyone come up with another scenario at all?
Existing dataset is a polyline, but doesn't have M values, which I'm going to be implementing. Else I would just apply subtypes to the existing dataset.

Cheers,

Cory