Should the inner sponge of the mattress be changed, or is it sufficient to change the mattress itself when there is a dry urine stain on the underwear before sleep?

If you doubt whether the mentioned spot is urine, then there is no need to purify it, because the default state is purity. There is no need to inspect, nor should one, because it increases obsessive thoughts (waswasa). If you are certain that the spot is urine, there is no need to change the sponge, the outer bedding, or the clothes. Rather, it is sufficient to wash the spot of urine on the clothes with water to purify the area.