Soda ash [Na2(CO3)] raises pH . . . but it also increased carbonate (=CO3) alkalinity a lot. High alkalinity is not a good thing, increasing scaling (and according to some data, corrosion) and decreasing the efficacy of both sanitizers and alkalinity.

So . . . feel free to use up the soda ash, so long as your alkalinity is low.

Otherwise, I'd recommend flushing it (literally) a little at a time.

Ben