People say that Spatarshi is a post and Brahmarshi is a person who understood the true meaning of Brahman. Can only a Brahmarishi become a Saptarishi?
Yes, you are absolutely right. All Saptarishis are indeed Brahmarishis. They were actually Brahma's mind-born sons (Manasputras). He created these mind-born sons, like himself. That's why they are all Brahmarishis. (Vishnu Purana I.7.1-5). However, not all Brahmarishis are Saptarishsis. For example, Vishwamitra became Brahmarishi by his effort. But remember one thing that Brahma had other Manasputras also apart from Saptarishis.