Answer to: Replacement HDD larger than 2 TB?Yes, you can. The A1409 contains a standard WDC Green 2TB HDD. I used the existing repair instructions to replace this with a WDC Green 4TB HDD - exactly the same except double the capacity. When the Time Capsule restarts you get an error message in Airport Utility saying the "Disk needs repair". Click the error message, then hit "Erase" and Airport Utility formats a 4TB partition on the new HDD.
Answer to: replacing 500Go with 2ToAre you absolutely sure that the wifi isn't up? You have done exactly what I did with a WDC Green HDD of the same specs. The light showed orange until I connected to the Time Capsule via Airport Utility and formatted the drive. If you can't connect via wifi, try doing this via an ethernet cable. By the way, 99.9% of English speakers won't know what a 500Go or a 2To is - you might use the acronyms GB and TB instead!