There won't be a significant length difference between a SU or Reloadable 24 mm casing.
The assembly drawing for the 50 NS AT F39 is available here.
https://www.aerotech-rocketry.com/c...s_assy_dwgs/24mm_mr_rms/24-40_f39-9t_assy.pdf and the casing drawing is here
https://www.aerotech-rocketry.com/c...ech_rms_ext_dim_dwgs/24mm_mr_rms/mr_24-40.pdf
A single 50 NS F39 propellant grain is 2.051" long, and the 24/40 casing length is 2.718" inches long. The new 24 mm 2-grain casing tube would have to be lengthened by 2.051" to make a 4.77" long motor casing tube for a 2-grain 100 NS G motor, and by 4.102" to make a 6.82" motor casing tube for a 3 grain 150 NS G motor.
The length of the a 24/40 motor 3.828" (97 mm) from the thrust ring to the ejection charge, so the length of the proposed 2-grain 24/80 motor would be 5.88" (149 mm), and the length of the proposed 3-grain 24/120 motor would be 7.93" (201 mm).
Bob