The following question might be a duplicate of How could different pronunciations arise when we are obligated to pronounce the Shema precisely?
Why did our Rabbonim however many hundreds of years ago allow changes in orening pronunciation in the first place? I presume if someone one day orened at the Omud and started pronouncing his ghimels and gimels the same or ayins and alephss the same way, he would have been told off afterwards by the Rabbi and that would have been it. Am I missing something here?