If I have a private key written down somewhere and for further security change one or more of the digits and then have the public address from the resulting private key written next to it. How secure is that? Would a brute force attack or any other attack be easier or no?
Now after you announced it; it is a security risk, but only if the attacker gets to know one of your private keys.
Then he will try to brute force the remaining ones by changing one or more of the digits.
Without disclosing any of your private keys, you should be safe; you can even use them in a sequence and it shouldn't matter.
I mean: assuming that there isn't any secret math behind ECDSA, that we don't know and they do.. which has been a concern.
I'm not sure you are understanding me. A regular brute force without knowing which digit or how many digits I changed is worthless since the total possible combinations should be exactly the same as a completely random number unless there is a relationship that can be derived from seeing a partial private key and a full public address next to it. That's the question here.