I haven't ever kept a marine tank but I'd think it would be an easy transition for a saltwater hobbyist to switch to keeping freshwater. You'd have most if not all of the needed skills and I would think a significant amount of the equipment could be re-used. It would be important not to assume the skills and habits are the same but surely they'd seem very familiar once you'd confirmed them.
Anyway, welcome to TFF and I hope you find some fellow travelers,
~~waterdrop~~