I dont think you will have to make any changes in development time if you are using the same film and developer. From what I understand the J&C film is the same, they just cut it in different sizes from a master roll.

Ah wait, are you going from tray developent to drum developent? if so, a good rule of thumb is about 15 to 20% less time, but it is still iffy, better try it with a negative that is not important.