Not sure if I get you right. If you turn your capital village into a city (such that the capital stays, but is only upgraded to a city), it is a city then, nothing else happens. Your fields stay the same level as before. The fact that you can upgrade your fields to level 12 instead of 10 only takes effect in side villages, not in the capital, because you can upgrade them indefinitely there anyway.
But if you switch your capital (such that your capital is now another village), then your fields in the previous capital will be reduced to level 12 (if it's a city) or level 10 (if it's a village).