Thank you for contacting Cypress Community Forum. You have to work with voltage as S29AL016J is 3V and S29AS016J is 1.8V. Please refer to S29AL016J and S29AS016J datasheet for detail specification.
I understand that they run on different VCC, but my concern is can they be considered as "drop-in" replacements? If I want to change this part on my design to S29AS016J (1.8V), would this cause any issues with timing, etc? I do have an existing design with the 3V part but due to some circumstances I need to switch it to 1.8V.
You are most welcome. Do you have any additional question?