Before I discovered you could just point your browser at http://search.cpan.org/meta/Dist-Name-0.007/META.json to automagically convert META.yml and META.json, I wrote a script to do it.
Anyway, it goes with my "I hate the cloud" prejudices :).

use CPAN::Meta;
use CPAN::Meta::Converter;
use Data::Dumper;

my $meta = CPAN::Meta->load_file("META.yml");
my $cmc = CPAN::Meta::Converter->new($meta);
my $new=CPAN::Meta->new($cmc->convert(version=>"2"));

$new->save("META.json");