Hi Dennis:
As I understand it, the JUpgrade extension will upgrade the core Joomla files to be compatible with the selected version – in this case, J1.7. It should not affect the content you have already customized – provided they were not core code modifications.
If the JUpgrade extension damaged customized content, then what would be the point of such an extension in the first place? If nothing was going to work and would have to be re-entered anyway, one could simply start from square one with a fresh install – right?
Nevertheless, I would check the information/documentation for the extension (or ANY extension) prior to using it – and/or contacting the extension developer to inquire as to your extension specific question . . . better safe than sorry.