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.
Private key is basically just many bits of binary (for Bitcoin private key it's 256-bit), there are many encoding or representation of private key (such as HEX, Base58 and Base64).
For example, HEX uses character 0-9 and A-F where lowercase or uppercase doesn't matter.