reuse instance - can't get format correct

This is likely a silly question but I can't seem to figure out the format....

I am creating a FileInfo instance that I want to reuse in a loop...changing the file that I'm looking for information on.  I did not want to create a new instance for every file.  This is what I have so far.....I don't have my loop created but don't worry about that.... I'll get to that... the problem is the line:  file = FileInfo(strBakFileName);   Basically, I'm trying to reuse instance of file and set it to a new file name and then do a file.Exists on the new file name.  I've tried file = (FileInfo)strBakFileName and that does work....

                FileInfo file = new FileInfo(strCommissionsPath);
                               
                if (file.Exists)
                {
                    string strBakFileName = strCommissionsPath;

                    strBakFileName = strBakFileName.Insert(strBakFileName.IndexOf(".csv"), intCounter.ToString());
                    strBakFileName = strBakFileName.Replace(".csv", ".bak");
                    file = FileInfo(strBakFileName);

                    if (file.Exists)
                    {
                        intCounter = intCounter + 1;
                    }

Thanks so much for your help!
                }