From Masterkey to subkey

[Bill Dangerous]

Hi,
Sometimes ago, I generated my master key without following the state of the
art of the gpg, meaning using an offline master key, and only sign and
enrypt with subkeys.
I would like today to correct my mistake without loosing my key, so I need
to transform this master key into a subkey. I followed this tutorial
http://security.stackexchange.com/questions/32935/migrating-gpg-master-keys-as-subkeys-to-new-master-key,
based on http://atom.smasher.org/gpg/gpg-migrate.txt. It works well, except
for subkey usage. Indeed, my old master key which is now a subkey, has all
the flags (SCEA), and I don't know how to change that. I would like to
limit flags to SE.
I tried to hack the code (
https://lists.gnupg.org/pipermail/gnupg-users/2009-May/036505.html), but it
works only for master key flags and not subkey flags.

I would like first to be sure that this process of migrating a master key
to subkey is reliable ? Am I not breaking something, that I am going to
regret ?
Is there a way (even if hacking gpg code is needed), to change those subkey
flags ?

Regards,
Bill
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