Workbook will not open in Excel 2007 but is okay in 2010 and 2013

I have been working on an order creation workbook for months. I started the workbook in Excel 2007 on my old computer. I got a new computer and installed Excel 2013 onto it in February. The workbook opened on my new setup without a hiccup. I have been working on the workbook in Excel 2013 for over a month, while a coworker using Excel 2010 routinely opens the workbook and enters information without issue. Today when I tried to open it on another coworker's computer running Excel 2007, it told me the workbook is corrupt and cannot be recovered. I went back to my computer and it opened fine. I tried on yet another computer running 2007 and it gave me the same corruption error. The workbook opens fine on the two computers running Excel 2010 and 2013 but fails on (so far) all computers running 2007. Does anyone have any idea why this is happening or how I might go about removing the corrupt data? I cannot feasibly remake this workbook from scratch. As far as I know, everything I have used in the workbook is compatible with all versions starting with 2007. I have tried manually repairing and extracting the data, but the extracted data is insufficient to remake the workbook.

I have confirmed that this workbook opens successfully on all computers running Excel 2010 or 2013 but will not open on any computers running 2007. All computers with Excel 2007 find "unreadable content" in the workbook and attempt to repair several times before finally declaring the workbook corrupt. How could it be corrupt if I can open it on my computer? Which part of Excel 2007 reads content differently than more recent versions? Any ideas? I really do not want to redo months of work. I have never run into this issue before...

Yes I can save it as a .xls file but I will lose functionality in my workbook. It shouldn't be necessary as no users are running a version of Excel earlier than 2007. .xlsx is compatible with 2007 and up so I don't believe that will work for me. I've already started rebuilding it unfortunately. I'm a few hours in.