The serial can't just stop working just because you update the OS, your Serial will always be valid, forever.
Be sure you are using the SERIAL Number, which is a long code that usually begins with FSDT. Do NOT use Order Number (STxxxxxx), or the SKU number. The "Register Serial" function only takes you Serial Number.
Be sure you are inserting them correctly, respecting everything: the hyphens, the case (they are usually all upper case).
Don't confuse the number 0 ( zero ), with the letter O.
Don't put extra characters at the end or at the start of the code.
Be sure you are selecting the product you need to reactivate first, then select "Register Serial", and be sure you are inserting the correct Serial for that product.
If Flight sim goes back to flying after you insert the serial, without doing anything, it means you inserted it wrong. You should see a request "Do you want to activate the product xxxx on this PC?". If you are getting an error like 25001 ( Serial Not found), it means you also typed it wrong.
If you see a black screen when selecting the "Register Serial" option, it means you haven't put Flight sim in a Window, that might be needed with some video drivers, as explained on the installation guide ( fsdt_install_guide.pdf ) page 15.
If you are getting an Activation error or a message that you have exceeded the maximum activations, in THAT case, you need to contact us via email, so we can reactivate you.
