I disagree, if you commit a crime eg take drugs, then that became legalised, and the criminal was in rehab (punishment) then would they release that person from rehab or would they have to finish the punishment?
You're right. But in your argument you're simply stating the claim that you serve the sentence passed to you at the time.
My argument is that Mercedes is willing to serve that and forgo the YDT as ordered. But why should they be excluded from an additional day of testing with race drivers testing a new compound tire for safety reasons?
You can disagree all you want but legally you're wrong and if Mercedes take this to the tribunal they will once again win their argument.
I have to side with Jabby here mate. You're argument has logic but the implication is wrong. As you say a fine itself cannot be directly changed - however, external sources can change the effect of the fine and that cannot be disputed by Mercedes (in this case) or the convicted party in any other example you give.
In the case of someone getting a fine, imagine you are Australian in the US and you earn Australia dollars. You are fined in April in a US court $1000. At the time, that means you pay AUD$950. When you receive the bill from the court you go and pay today (July) and you have to fork over AUD$1100. You have no case to complain as the currency fluctuations are extraneous to the fine.
The changes to the YDT - longer time and team drivers being allowed to participate have been necessitated by the generally accepted external changes to the landscape (tire problems from Silverstone).
All the complaints from the Mercedes fan-base may be understandable, but, as I mentioned previously, if the YDT went ahead and it poured down with rain and little testing was completed then that would just be bad luck for the teams testing.