Simba's SP design isn't that significantly different other than the color scheme. Size-wise, he's actually about the same as in the first film. He's pretty much kept at around the same height and bulk when one compares scenes of him sitting near Rafiki from the original and the sequel.


And the smaller things like the amount of mane fur he has compared to before isn't really something worthy of note or an intentional change, as it's inconsistent throughout both of the films. Sometimes it looks thicker(as shown in the above comparison), sometimes it looks the same. Sometimes it looks even less thick than in the first movie, as shown below:

It's mainly that darker scheme that gives a more mature-vibe to him. The rest is all due to differentiating artists and his bulk/mane thickness fluctuating during various shots.
As for what direction The Lion Guard will go, the answer is neither-ish. Given that the show seems to have a distinct art style, it's a given that Simba will probably be drawn a little differently than before. HOWEVER, regarding his color scheme, he'll no doubt be rendered more faithful to the original film than Simba's Pride. His original yellow/gold and red look is more iconic and remembered, whereas the SP colors were unique to that feature's general art design and palette they were going for. In-universe, Simba's colors didn't actually change as he got older, their alteration is just a by-product of the film having its own distinct tones and not being consistent with the original feature. A number of Disney DTV sequels similarly had different color schemes for their various characters.