使用PowerShell编辑XML并出现“文件格式”错误

I am using the HyperV Module from Codeplex to do a "config only" export from a 2008R2 Hyper-V server. In order to import the configuration on another HyperV server, I need to edit the value of CopyVMStorage in the EXP file. This file is an XML file. I wrote the following code in PowerShell to do the update for me. The variable $existing is the existing exp file.I am using the HyperV Module from Codeplex to d