Switching any kind of pellet brands etc. should be done very gradually with a small dilution of the original pellet. Timothy pellets are actually much better for an older rabbit because they are not as fattening and do not contain so much calcium which can cause bladder and kidney stones. The amount of pellets you are feeding them both is a little too much. What you want to do is supplement with a variety of veggies. There are a few feeding charts available online and do vary a little. This is the one I follow:
Here is a chart for your older bun from this web site:
https://binkybunny.com/Default.aspx?tabid=60
This is for your "young adult" bun that is 8 months old from the HRS:
http://www.rabbit.org/faq/sections/diet.html#young
You may want to split the pellets in 1/2 and feed 1/2 in the morning and 1/2 at night (I do). Gravehearted gave you a great tip about measuring out how much your bun is actually eating.
Here is a good article about timothy based and alfalfa based pellets:
http://www.wisconsinhrs.org/Articles/Pellet%20Types.htm
If your older bun is not overweight and is so sensitive and has not health problems then I think you are OK with remaining on the same pellet. If you do decide to switch the pellets with your bun;
This is a 4 week process that I’ve taken from Oxbow:
Week 1: Mix 1/4 of new pellets with 3/4 of the food you usually feed
Week 2: Mix 1/2 of new pellets with 1/2 of "old" food
Week 3: Mix 3/4 of new pellets with 1/4 of "old" food
Week 4: Feed 100% of new food