So what I’ve done to reuse a resin tank was to scrape as much of the old resin out using the scraper provided in the cleaning kit. I would scrape the resin into a corner of the tank so it would pour out smoothly. This was advised by Formlabs support. I made sure to scrape out what I could from all of the sides and bottom surface of the inside of the tank. It’s a really tedious process, but now I switched from clear resin to white resin with the same tank.
The printing results aren’t the greatest as I need to avoid using one corner of the tank in order to get successful prints. Having that in mind, if you really are in need to reuse one of your tanks instead of just buying a completely new one, then it is definitely possible.
Ultimately, I think the best solution really is to use a separate tank for different resins. I’ve only switched from clear to white - I cannot speak for any of the other resin types.