To answer my question, I just called to apply and asked if they could convert my existing platinum card to the signature. They could not, so I just went ahead and applied and will cancel the platinum.
I used the extension for the 25K miles, then another 5K if you spend $500 by Aug 31. The rep also confirmed the annual fee is waived for the first year. No instant approval but I will receive in 7-10 days by mail.