My main question is, does it make a difference on a clear ECDSA private key if i put the letters all in Upper or not because i realize that on the side of "tp´s go bitcoin tests" where i only look how a real key looks like. all is in upper letters.
For private keys in raw hexadecimal format (containing the characters 0-9 and A-F), then it makes no difference in the A-F characters are upper or lower case.
If the private key is in any other common encoding such as WIF or minikey which use Base58, or in Base64, then it is case sensitive.