My 32 weeker had terrible stomach issues after every formula feed and unfortunately I produced very little breast milk so I started introducing really runny solids such as full fat yoghurt and baby rice (made with breast milk or 50:50 formula and breast milk) quite early - 5 months (actual age). I'd tried everything before that - different and lactose-free formula, all and every anti-colic medicine, anti-colic bottles etc.
The difference after introducing purees and runny solids was remarkable. His doctor-diagnosed "colic" (which was clearly just wind issues caused by fortified milk being unleashed on an immature stomach) disappeared and feeds became much mire relaxed.
I know all babies are different but he was so ready for food (or rather ready to abandon the formula) and weaned beautifully and fattened up a treat. I slowly replaced one meal at a time until he was almost entirely on solids at 7 months. He abandoned formula entirely of his own volition at 9 months and I offered him cows milk around 12 months which he's lived since. That 3 months without milk made me nervous but I needn't have worried as he stuffed his face with everything else to make up for it!
Loads of mums kicked off at me about giving him food too early ("milk is best", "he'll develop allergies", "his stomach can't take it") but I know my child and clearly formula wasn't doing him any good and seeing as I couldn't breastfeed (long story...), it made sense for us to wean early.
He's now a healthy, happy three years old, eats everything and has no allergies.