When I create equations, and then save the document as a Word doc (1997-2003), and then close and re-open the file, all the equations look different.
As a simple example: I created an equation “a = b + c”. After close and re-open, the formula looks like: " size 12{ ital “a=b+c”} {}" . This is a simple case - more complicated formulas become much worse.
How can I prevent this?
You can make nothing. It’s working so. Just use default ODF format for saving of your document
Just to reinforce the answer by @kompilainenn , save your documents in a proprietary format (like MS-Word .doc) only when you need to send a copy to someone who can’t read ODF’s open formats. Otherwise, always save in LO’s default formats (like .odt). By the way, I understand (although I could be wrong) that recent versions of MS-Word can read .odt format.
Not wanting to “muddy the waters”, but the answer to this question might also give you some insight :
The key thing is to avoid conversions of format which cause errors, such as you have already found.