Titleist is not allowed to give out pre-release information so officially, you will have to wait and see. Do not expect a formal answer to your question from Titleist. Sorry. My personal belief is that they will release a T300 and T400 again this cycle, which would mean late fall, winter, or early next year. I have no special intel, just my opinion on how successful those clubs have been to the general public. I would not expect the T300 lofts to change as they are already strong compared to the other models.

From what I have been told, the MB, CB and T400 are remaining "current" in the line after the new T series launches - meaning you can still order them for awhile longer. The original plan for the 620 mb and cb was that it would be a 4 year cycle. If and when the next version of the t400 is launched appears to be a ways out still.

The T400 is not just a T300 with a stronger loft. You can look at the sole and see the split design to keep the iron from interacting with the turf with a wide base. So I agree the T300 will likely keep the same lofts and since the T400 was only released a year ago and is not tied to fit in-between the T100 and T200 like the T100-s, I also agree it won't be updated with the fall release this year.
Compared to the T300, my T400 launches like my AP1 7 iron and goes as far as a T300 5 iron. It is simply a different design but works well at my overall slower swing. With the 45 g shaft, I'm sold on staying with my current T400's. I did have to add the T400 43 degreee to fit between the 38 degree PW and my 48 degree gap wedge.