Write to .xlsx file on older computer

Is there a way to write a macro that will access a .xlsx file on a computer that has only Excel 2003 installed?

I can't resave the file as a .xls, it's a file made by a client and it's too big with too many conditional formats to do that. I've tried, and it kills my computer.

The error I'm getting is in this code:

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Oddly, this code from a module that runs beforehand works fine:

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I believe it may be due to the fact that this file opens Read-Only because 2007 is not installed on the computer I'm trying to run this macro on. However, I may be wrong, and I would appreciate any thoughts anyone has.

Thank you.

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