Different rent for each lease term option

[Kristoff Hendrickson]

I'm a new duplex owner and first tenants are approaching the end of one-year lease.  They are good tenants and I want them to stay.  I'm planning to offer a one-year renewal without a rent increase.  Tenants are a young couple that may be looking to buy a house in the next year and I don't want to drive them away with a one-year or nothing lease.  However, I would also like to protect myself from an earlier vacancy by increasing the rent for a 6-month term and increasing further for month-to-month.  For example, $1,400/month for a one-year lease, $1,450/month for a 6-month lease and $1,500/month for month-to-month lease.  Is this type of renewal structure legal?  Duplex is located in Minneapolis, MN.

Thanks!