When the Wizards of the Coast decided to radically change the rules of D&D from 3.5e to 4e, they also changed the Realms to suit the vision of the new edition. In 4e, there was a drastic reduction in the size of the FR pantheon, with a lot of the deities disappearing. One of these changes was Lathander being replaced by the old Netherese god Amaunator (of LN alignment).
Possibly as a preparation for this change, the 2006 sourcebook "Power of Faerun" describes two "notable" heresies within the ranks of Lathander clergy. The first is called the "Risen Sun Heresy", perpetrated by the Brotherhood of the Glorious Sun. Supposedly tolerated by other followers of Lathander, those who follow this belief claim that Lathander is just a re-incarnation of Amaunator. The second heresy is the "Three-Faced Sun", which supposes that the Sun is a tripartite overdeity, with aspects of dawn (Lathander), highsun (Amaunator), and dusk (Jergal/Myrkul).
An important in-game event happened in 1374 DR: A high priest (Lathander's archbishop of the Dragon Coast) cast an epic spell called "Amaunator's Eternal Sun" in Elversult. This effectively caused something like a second miniature sun to appear on the sky that could be observed within a 150-mile radius. It rallied many to embrace Amaunator's return. If the date of your game is around 1490 DR and your players come close to Elversult, they could meet some people (like elves, dwarfs, gnomes, etc.) who must have first-hand memories of this event. Note that Suzail, the capital of Cormyr, appears within 150 miles of Elversult.
Anyway, it looks like all of the "heresy" material were a preparation for the 4e update of the Realms, as R.A. Salvatore describes in a 2013 interview how massive changes about the campaign setting were revealed to him and others (including Ed Greenwood, one of the authors of Power of Faerun) in 2006. (You can watch it here: https://www.youtube.com/watch?v=aLf1hBUr9M4, particularly around the 13th minute).
In 4e, Amaunator was described as a LG deity, so it looked like he had indeed absorbed Lathander in some way.
Eventually, Wizards must have realized that the 4e changes to the FR timeline were not welcome by the fans of the setting. With the event called the Second Sundering, a very large portion of the 4e changes were reversed. Lathander returned, while Amaunator became LN again. The retcon is such that claiming that the two are one and the same has always been a heresy, but in reality they have been separate. (Or they were one for a century and then got re-separated; feel free to choose according to your own campaign.)
Update following a comment by @tardigrade: There are conflicting data from the official sources regarding the Amaunator's Eternal Sun spell and its effects. While according to the PoF, this epic spell was cast in Elversult (and its duration is permanent), the 4e FR Campaign Guide lists the "second sun" located above the skies of Elturel. Moreover, the PoF version hides the normal sun, while the FRCG version can be observed in addition to the real sun. According to the discussions on the web, part of these discrepancies can be attributed to Richard Baker getting confused about the cities. We might need to retcon the discrepancy as the miniature sun following its caster as he goes from Elversult to Elturel.