I'm assuming that "AMP format" is a modified form of HTML? If that's true, see if something like this works for you:
$HTML = New-Object -Com "HTMLFile" $src = Get-Content c:\junk\x.html -Raw $HTML.IHTMLDocument2_write($src) $HTML.documentElement.outerHTML | Out-File c:\junk\NewX.html
Note that using COM is known for being persnicketie, and the HTMLFile COM object uses (IIRC) the Internet Explorer HTML parser -- so be prepared for possible parsing problems!
Another choice may be the HTMAgility package . . . it's not something I've used but it seems to be better than that COM stuff. Here's an example using PowerShell: html-agility-pack-rocks-your-screen-scraping-world