I've been head-canoning that the majority of Trill are spotted, and the forehead-Trill are a subspecies from one smaller, Australia-sized continent. Us Humans used to have different subspecies, such as the Neandertals, but we homosapiens are the only ones alive right now (unless Neandertals bread into us, but anyway).
I'm actually working on a fanfiction that will touch on the Trill makeup change, using my above stated head-canon. I'm calling them the Horvati Trill, after Michael Horvat, writer of the TNG episode "the Host." When Beverly met Odan, the Horvati Trill were in charge of the Trill government, and had different views on joining than the rest. The Horvati believed that the host should be subservient to the symbiont, and deliberately chose weaker-minded people to be hosts.
After blowing the secret of joining to the Enterprise, the Horvati lost control of the Trill government, and the Spotted now rule the Symbiossis Commission, with the values and priorities shown on "Deep Space Nine" ("host and symbiont must find a balance so one doesn't overpower the other...")
Horvati Trill still exist, but many non-Trill mistake them for another bumpy-forhead race, and don't realize that they're a subspecies of Trill. I don't know if this will ever come up in my story, but I do also head-canon that Verad's girlfriend Mareel (DS9 "Invasive Procedures") is a Horvati Trill. Her forehead isn't exactly like Odan's, but it's close enough, and Klingons have plenty of forehead variety.
On the subject of makeup changes, my past fanfics have also used the idea that Ktarian foreheads vary even more than Klingons', which is why Naomi Wildman is blessed with cute horns and spared the unfortunate facial features that Etanna Jol on TNG's "the Game" had.