I know changing substrate is for keeping the enclosure clean. But do I still need to do it when there are a big number of springtails inside the substrate?
It’s really best and more economical to “spot” clean, ie, routinely/daily check out your torts’s enclosure removing old food, animal waste, etc. Then, every once in awhile, remove some substrate and add new substrate back in. But, there’s no reason to just willy nilly toss and replace. Depending on the size of your enclosure, that can get very expensice fast AND it also disorients your torts environment when done.
Conduct daily spot checks, remove waste, add new substrate occasionally, and let your springtails earn their keep!