New Home Came with In Ground Salt Pool - SO much to learn!

My husband and I purchased a home which has an inground salt water pool. It's an awesome pool, rectangular in shape and the deep end is only neck deep so we use the entire pool. From what we can figure out, the pool volume is roughly 16,000 gallons.

The salt system is Hayward Aquarite. We're going to replace the salt cell because a) we have no idea how old it is and b) based on how old the pool's heater is and how poorly that was maintained, we don't have a warm fuzzy about the salt system.

The T-Cell in the salt system is a T-15 Turbo Cell, rated up to 40,000 gallons. Do we need this? Before I went to look at the T-Cell, I was looking at the T-9 cell online, which is rated up to 25,000 gallons which should more than cover our pool. So we were a bit shocked to find that the T Cell in the system is a T-15. Can we replace the T-15 with a T-9? Should we?

Sign us clueless but anxious to learn!
Thanks,

Lisa