The FM16W08 logic levels are CMOS levels. Therefore, the logic levels will differ between FM16W08 and FM1608D . VIH is the key parameter to be looked for carefully in this case because it becomes minimum of 3.5V for 5V CMOS operation vs 2.0V for the TTL. That means, the controller driving a logic level HIGH below 3.5 V will be OK for the TTL but will be treated as lower than min spec for the CMOS, hence logic detection will not be gauranteed by the FM16W08. However, if the controller output is driving its I/O to rail to rail (0 to 5V), in this case FM16W08 can replace FM1608B.
Please let me know if this helps.