Find files by extension and replacing characters before and after the search pattern

I need to find a files by extensions and replace the random characters before the search pattern to the specified text (REPLACED), turn the numbers after this pattern into an increasing counter and move 'em to another folder. Examples below: find files by extension .EXT

/opt/files/QvmBIsB3_PATTERN_77580.EXT
/opt/files/8iV8QhFwQos_PATTERN_77580.EXT
/opt/files/lgI6JUEh55za488_PATTERN_77580.EXT

change 'em and replace to another folder:

/opt/replaced/REPLACED_PATTERN_1.EXT
/opt/replaced/REPLACED_PATTERN_2.EXT
/opt/replaced/REPLACED_PATTERN_3.EXT