When I tried upgrading they would not allow it. I had only had the Signature Visa for 6 months. They claimed it had something to do with the new credit card regulations. They gave me 2 options. One - to wait until the card was open for a year and then I could upgrade. Two - to close the Signature Visa and reapply for the Select.
Of important note is that they claimed I would *not* be eligible to get the bonus miles on option 2. However, that is the option I went with and the bonus miles posted. However YMMV...