The conversion kit for the small pot will reduce the amount of media coming out of the pot. Soda is really small compared to sand or glass beads. Even with the conversion kit it will still be putting out more soda than you need to do the job. More soda out means more $$ to do the job.
The big pot could get a suitable metering valve but it would still be putting out to much soda. If you want to soda blast then get a pot designed for soda, adjust pot pressure and line pressure for fine tuning soda flow. A good soda pot with hose and control valves and a wet head is around $6600 in addition to the pot you will also need a cooler/dryer package $$not sure. These units will make the job go much better and more profitable.
Keep the sand pot for using other media, corn, sand, crushed glass, etc. I do a good bit of corn blasting and tryed using soda through it with the metering valve almost closed and it did a good job but used about 3-4 times the soda I should have and lots of soda dust, dead plants.