I had houseruled something like this for my game a while back. No one ever managed to take advantage of it though.

I had some heavy restrictions.

1. You had to increase a province to 10. Not all provinces are suited to supporting a metropolis.
2. The maximum number of trade routes had to be established for the province in question.
3. The province must have a castle level equal to at least 4.

I had some other restrictions I can't remember, but those were the big ones.

You then had to pay a price as if you were increasing the province level to 11. Normal rules for increasing province level apply. This includes checks and challenges.

If successful, you now have an Urban Province level 8 and a normal province level 3. Trade routes and holdings would then need to be split between the provinces in whatever way seems most appropriate. GM adjudicated of course. The castle level would stay with the city.

If you don't like that, then leave the city at level 10, and you gain a level 1 provice around it, but that doesn't make as much sense to me.

In either case the maximum level of the normal province would be permanently reduced by one.