You do not have to store a symbol on disk to use it in your current model or drawing. However, you must store the symbol on disk for the symbol to be available for other models or drawings or for other users.

Storing a redefined welding symbol will not affect instances of the symbol already in models, or subsequent instances added in models that already contain instances of that symbol. The redefined welding symbol stored on disk will only be used for new instances in models that never used that symbol previously, or in models that previously used the symbol and you specifically retrieve the redefined symbol from disk. This overwrites the symbol stored in that model and updates all instances of the symbol in the model to the modified symbol.