irule to replace any .asp to be .aspx in uri

I apologize, but beyond the .asp to .aspx part, it's not entirely clear how else you want to redirect. However, assuming, I'm assuming you want the following:

if host == 123.abc.com, then
  if Request Target Path ends_with .asp or .aspx, then 
     redirect to www.abc.com[path][query-params], with .asp changed to .aspx
  else redirect to knowledge.cibc.com[path][query-params]
else
  do not redirect

Adjust the following as needed for your actual requirements:

when HTTP_REQUEST {
    if { [string tolower [HTTP::host]] eq "123.abc.com" } {
        switch -glob "[HTTP::path]" {
            "*.asp" {
                HTTP::respond 301 Location "https://www.abc.com[HTTP::path]x[HTTP::query]"
            }

            "*.aspx" {
                HTTP::respond 301 Location "https://www.abc.com[HTTP::uri]"
            }

            default {
                HTTP::respond 301 Location "https://knowledge.cibc.com[HTTP::uri]"
            }
        }
    }
}

Notice that I cut your default branch, which made a pool call. If the pool assigned the Virtual Server is already /PROD/po_abc there is no need to explicitly identify the pool here. If the rule doesn't change the pool selection, then the pool assigned to the Virtual Server will automatically be used.