use "old" masterkey as subkey in new masterkey

[secretaries_splenoptosia at aleeas.com]

hey, i am trying to manage my protonmail gpg by adding it to my masterkey. I have looked at https://security.stackexchange.com/questions/32935/migrating-gpg-master-keys-as-subkeys-to-new-master-key I was able to slot in my subkey, and was able to concatenate it into one file as well, but while importing it (with the lastest version of gnupg) i got an error - bad signature, realizing that i had to use an old version of gnupg - around 2005, but I did not have access to any such gpg binaries. Are there any workarounds for this? adding a subkey with keygrip also doesnt work, since the new subkey has a different keyid than the original key. Can someone help me?
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