Nature
2 must be used with chlorine, not bromine, so you need to change the water. If purging the lines is easy for you to do, then do it, but there's probably not enough water left in them to be an issue with the leftover bromine/bromide. If it's only a small amount of bromine (or bromide), it will eventually outgas when you chlorinate the spa.
I presume that with Nature
2 that you want to use it with non-chlorine shock (MPS) except for once a week or so chlorine. If so, then you need to maintain an MPS level so need to get test strips for that (or use a supplement to the Taylor test kit, but it doesn't sound like you are using that). See the
Nature2 Spa Owner's Manual.