Gaining

I've gained about 25 pounds so far and found that I needed to start replacing clothes at about 15-20 pounds. Beyond that I've had no issue, though idk if there are differences with a HC gain.

Everyone needs to be careful if you are gaining. I know of one woman who was average size and thought it would be nice to gain 50 pounds or so. She is now closing in on 600 pounds, and is still gaining. She is not gaining on purpose. She just can't stop it.

I know it’s not a satisfying answer, but there’s really no way to know. Some people can lose that amount of weight easily without even trying, others can’t even with great effort. That being said, though, unless you’re very short, 10-15 pounds on a thin person is an amount that’s likely to feel like a big difference to you but not be very noticeable to other people looking at you, so being unable to lose it would probably not be a huge deal.

www.ncbi.nlm.nih.gov/pmc/articles/PMC2936436/

Here is real life experiment with a modest sample of 18 people who ate a bunch of fast food to intentionally gain weight for science. I personally find this very hot to just read for fun but you might find it genuinely helpful. On average participants' BMI increased from 21.8 to 24 during the pigging out phase. When they went back to normal life, they initially lost about half the weight they gained, but by two years later they were almost the same size as their fattest, which was still not overweight.

This suggests that yes, gaining 10-15 lbs is likely to shift your set point up a bit metabolically. If you want to go back to your starting weight and maintain that long term, you might find that harder to do. It's also likely that there is some selection bias here because the only people who would consent to be human subjects in a fattening experiment don't mind the idea of gaining weight. So maybe the people in this study either intentionally kept the weight or just didn't really try to lose it.

On the other hand, if your concern is harm to your health as opposed to being just a bit thicker than you started, it's probably not a huge difference but everyone is different. No matter your size, you can try to stay active and eat lots of plants and minimal sugar.