Yes, you can. It depends on the router. If you have updated it post FCC firmware compliance then your router could be locked after updating the firmware. My Linksys EA6500 got locked. Trying to change it to Tomato or DD-WRT just results in an error. My Asus RT-AC56 apparently is locked from the web ui, but maybe possible through the emergency update routine. I haven't bothered to flash it with ASUS firmware and have stuck to Merlin's. I have no need past what Merlin provides anyhow and Merlin sticks to ASUS firmware as a base, minus the FCC lock. I know for a fact that the DLinks also use a tftp emergency update routine on bootup so those may still be possible to update to 3rd party firmware.
They aren't always clear with the release notes, but I'm sure you can read between the lines.