Conversion of profile username to lower case

I'd suggest the lower() function - It takes any text and converts all the characters to lowercase.

20.3 we got a a!renameuser to perform the operation . but can we have a rule which could get us all id's on the env with starting character as uppercase? 

You'll have to use the function getdistinctusers() to return the list of all the users in a group. Then, you can use the exact() function to find items that don't match the casing that you're looking for. Something like this should work:

reject(
  fn!isnull,
  a!forEach(
    items: getdistinctusers(
      cons!GROUP_NAME
    ),
    expression: a!localVariables(
      local!firstCharacter: left(tostring(fv!item)),
      if(
        exact(lower(local!firstCharacter), local!firstCharacter),
        null,
        fv!item
      )
    )
  )
)

You'll have to use the function getdistinctusers() to return the list of all the users in a group. Then, you can use the exact() function to find items that don't match the casing that you're looking for. Something like this should work:

reject(
  fn!isnull,
  a!forEach(
    items: getdistinctusers(
      cons!GROUP_NAME
    ),
    expression: a!localVariables(
      local!firstCharacter: left(tostring(fv!item)),
      if(
        exact(lower(local!firstCharacter), local!firstCharacter),
        null,
        fv!item
      )
    )
  )
)