A lot depends on whether (a) the T7400 motherboard will fit in a T7500 case, and (b) if the connections to the front panel are the same on these 2 models. I don't know the answer to either question.
Dell typically uses custom connectors between their motherboards and the front panels. They don't make that info publically available. So unless they're exactly the same connections, or you or somebody else can map the connectors, this may not be doable.
Have you searched these forums to see if anybody has already done what you want to do?
Forum Member since 2004
I am not a Dell employee