This sounds like a job for a commit trigger.
There is a 'changed' hash - which has the old field values in it (the EPrint object already has the newer values in it).

You can check whether the field you are interested in is a key in the changed hash, and use that as a basis for sending an email.

NB The example below may not be syntactically correct. It's lashed together late at night (for me anyway), but I think you'll get the idea. Happy to answer questions about it in (my) morning!

Cheers,

John

$c->add_dataset_trigger( 'eprint', EPrints::Const::EP_TRIGGER_BEFORE_COMMIT, sub

{

    my( %args ) = @_;

    my( $repo, $user, $changed ) = @args{qw( repository dataobj changed )};

   if( exists $changed->{'the_field_youre_interested_in'} )

    {

        my $old_val = $changed->{'the_field_youre_interested_in'};

        my $new_val = $eprint->get_value( 'the_field_youre_interested_in' );

        # generate an email alert

    }

}, priority => 73);