Hi V,
The most common cause of hat error message is attempting to open with Numbers '09, a Numbers file created in or last saved from Numbers 3 or later. The file format used by Numbers '09 used (and required) an INDEX.XML file as part of the package that was the .numbers file. Versions after Numbers '09 (version 3 and later) use a different file format that did not require (and did not produce or include) an INDEX.XML file.
As Numbers versions for iCloud and iOS were introduced and syncing between devices became easier, any Numbers file that passed through an iOS device or iCloud was affected by the change in file types.
If you have Time Machine running (or some other backup system, you may be able to recover an earlier version of the problem file(s), and proceed from there—isolating the file(s) from iOS and iCloud versions of Numbers.
Fixing the files, assuming I've correctly diagnosed the problem, would require finding someone running
Yosemite or El Capitan (or later version of OS X) and a version of Numbers capable of opening the files as they are now AND exporting them to Numbers '09 format, or to .xls (Excel format, then returning them to you to be opened in Numbers '09.
Moving ahead—There's a large gap between Lion and Catalina. Perhaps the highest cliff is the Catalina requirement that all applications must run in 64bit mode. For most, if not all the applications on your iMac, written to run in 32 bit processors, that means they would need to be replaced if you wanted them on the new computer.
Regards,
Barry