Please you can refer to https://www.cypress.com/documentation/product-change-notice-pcn/pcn154205
The silicon revision A,D are actually refer to mask revision, the mask revision change will result in improved HCI (Hot Carrier Injection) performance of devices in certain applications that use UART or USB protocols utilizing the Internal Main Oscillator (IMO) of the device.
Functionality and specifications remain unchanged from the original design. This mask revision change will be applicable to select Industrial and Commercial grade PSoC 1 and USB enCoRe III products manufactured at the CMI (Cypress Semiconductor Minnesota Incorporated) fab in Bloomington, MN, and the HHGrace fab in Shanghai, China.
The Mask Revision at CMI is 'A', the new product is 'C'.
The Mask Revision at HHGRACE is 'A' and the new product is 'D'.
The 'D' revision CY7C64215 can be used no problem with your design which works with 'A'.