Reported speech modal verbs could and might

I know that usually we don't change modal verbs in reported speech but i want to know if it is OK to change could for might as it doesn't change the meaning. I failed my English exam on this one question and my teacher says that I can pass if I can prove I'm right.

"I could be very rich when I'm 35." said Tom
Tom told me that he might be very rich when he's 35.

I don't see the problem as it means the same. The rules say that modals don't usually change, but they don't say that they never change.

Help needed urgently