CO Landlord - Tenant wants to change their mind about moving out.

[Virginia Lacy]

Hi everyone, long time BP reader/ first time forum poster. 

I'm a Colorado landlord. I have a tenant who re-signed a year long lease and a few weeks later informed me that they would be moving out on August 1st. I informed the tenant that a lease was in place, but that I would work with them to find a new tenant. I listed, arranged showings (the current tenant did help some) and vetted numerous candidates and found one for the unit. During this whole process the current tenant verified multiple times in writing that August 1st was their planned move out date. 

Yesterday the current tenant contacts me, tells me that their plans have changed and that they would like to stay. This tenant has been relatively high maintenance during their whole tenancy (year and change). The only reason I leased the property to them for another year was that I am working out of state, and they had been relatively well behaved during the months towards the end of the first lease. 

Am I on good legal standing to insist that the current tenant vacate the property by August 1st since the lease was amended (at the behest of the tenant) and agreed to in writing by both parties? 

Thanks for any advice! 

-V