The simple answer is "yes." In principle, of course people can go from primaries to secondaries, especially if those labels are using more descriptively rather than setting expectations for how the relationship will be.
The complicated answer is "not always."
In my experience, incompatible is incompatible. So while I can understand something like "I can't stand the way he spends all his money the day he gets paid, so I can't share a house with him because I'm always stuck paying the bills" being a good reason to take the relationship intensity down a notch, I have trouble understanding how that would look for different values about sex. Perhaps if you explained what you mean by "values" in that context, it would help me understand.
More concerning to me is the idea that you cannot be healthy in a primary relationship but that you could be healthy in a secondary relationship. Again, this could be a problem of understanding.
End of the day, you can try anything you'd like. What works for one person may not work for another, and you'll never know until you try. Just take it slow and be aware of your feelings and needs and make sure you're taking care of yourself.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker