changing endian in Avalon FIFO Memory

This is because by default, avalon FIFO's streaming interface has the properties "firstSymbolInHighOrderBits" set to be true. You can check it in Qsys>System contents by clicking on the streaming output. When it is true, data[31:24] will become D0 in data streaming https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/manual/mnl_avalon_spec.pdf, Table 18. You could not change it as it is predefined setting in the IP. What you can do is to reverse the data symbol at output of Avalon-mm FIFO.