Yes, you can easily switch between investment models and yes you can hold more than one investment model in the same account however you can only hold one diversified portfolio per account. If you wish to invest in multiple diversified portfolios you will need to hold multiple accounts.
To switch between investment models simply click on Modify Allocations via the Quick Links section or within My Account. There are no administration fees for changing between models. The only costs associated would be the brokerage charges for selling and buying the new holdings.