Modifying themes while keeping hacked custom fields?

Hey all, I’ll try to keep this short and simple.

Let’s say I modified the default theme’s index.php, comment.php, and sidebar.php to include some information generated from a custom field, using get_post_meta().

Except now, I want to suddenly change to, say, classic theme. I then lose all those hacks I inserted into default theme.

Is there any way short of entering classic’s files and reinserting the hacks that I can get the custom field information back?