Why did value of C6 change from 1uF to 2.2uF beginning from rev 3 of the MAX3250 datasheet? Do the designers need to make revision in their designs?

In Rev2: 1/08 of MAX3250 data sheet, C6 is specified to be 1uF. In Rev3: 9/11, and Rev 4: 2/15, C6 is specified to be 2.2uF. Maxim now recommends a 2.2uF capacitor for C6 to further noise reduction and improve performance. There have been no changes done to the silicon. 

Ultimately, it is the designer's decision whether an existing design which used a 1uF capacitor should be changed to 2.2uF. In most cases, no change is required.