If I Just Get Liposuction on my "Flanks" Will the Fat That Usually Goes There Go to my Butt?

Only a surgeon with a trained aesthetic eye who understands the underlying muscular anatomy can best answer this question for you. The trunk is an aesthetic unit consisting of the abdomen, lower back, buttocks and flanks. They all must be in proportion. When looking at the buttocks, there are two issues to address when considering fat grafting: is there adequate fill and are there issues with the shape. Re-shaping a buttock contour can eat up a lot more of your graftable fat than augmentation. You may think your buttock needs no grafting, but in fact, you may benefit from it. I would recommend that you find a surgeon who is very experienced and comfortable with these issues, and best if they perform 4D VASER liposuction and grafting as the VASER is very gentle on the harvested fat. Good luck!!

liposuction doesn't affect your future ability to gain weight or lose weight. If you gain weight after lipo in the flanks, your body will preferentially place it in other untreated areas like abdomen, back, butt, breast, arms, inner thighs, outer thighs. Some of it will go to the butt.
the best thing is to try not to regain the weight back.