I only have primary aged DCs so different from the OP and my answer would probably be different once they were teens or started taking an interest in what they wear, but I only replace uniform when it is grown out of, not if it is stained. If I replaced uniform every time there was a new, permanent stain I would be buying new polo shirts every week! Our school uses a lot of white board pens which stain the yellow polo shirts horribly.
I dont hand down to the next child a stained item, but otherwise it is kept in use, with tears sewn up/patched until it's out grown.
If uniform is truly grown out of (literally child can no longer do it up/get it over their head) during the year it is replaced and in the summer an inventory is taken new items bought as appropriate.
I am sure my strategy will change as the DCs get older and mind more how they look, but for the moment I don't have money to spend to replace uniform that is still usable. I view uniform for infants-aged children almost like overalls that protect the other clothes that I wouldnt want them to wear when painting, using white board pens, wading through mud on a field etc, and I wouldn't replace them just because they were no longer in top condition!
Sorry OP this isn't relevant to your query, but I thought I would add my experience for the primary parents reading.