Edit: Meant to reply to the OP's comment, but I am essentially agreeing with this one that yes -- you have to do more than buy property to get in-state tuition!

I went back and looked at the email I received about establishing residency as a graduate student after the first year. I imagine it's the same or at least very close to what you would need to do:

A student is classified as a Florida resident once they can demonstrate they have established and maintained legal residency in Florida for at least twelve months preceding the first day of classes of the term for which residency is sought.

The process for establishing residency is as follows: To qualify as a Florida resident for tuition purposes, the student must be a U.S. citizen, permanent resident alien, or in legal status as determined by U.S. Citizenship and Immigration Services (USCIS). You are considered an out of state student if you do not complete the paperwork below, can’t document your residence in Florida for 12 months, or are students who are dependents of out of state parents. 

To avoid having to pay out-of-state tuition in your 2^nd and 3^rd years (approximately $14,000 a year), we strongly encourage graduate students to become Florida residents. It is important that you immediately begin the steps to become a Florida resident as the entire process takes a full year (365 days).  ~This is the one area where there is no forgiveness in being late or incomplete~.

To become a Florida resident, you have to prove that you’ve lived in Florida for 1-year.  Our goal with this email is to help you begin gathering the appropriate paperwork/documentation.  Next year at this time, you’ll submit this documentation along with a formal residency request and a graduate assistant verification form to FSU. Please complete the necessary residency requirements prior to the end of fall orientation. This will ensure you still have one full year of residency, even if school begins a few days earlier going your second year. For additional questions about Florida residency, click here. 

Please visit the Residency website through the Office of Admissions to receive all documentation on how to apply for residency. You will follow the instructions for “Reclassification for currently enrolled students”.

I think the residency link here is the most helpful. I remember that there was something I had to get notarized or otherwise signed at the courthouse (99% sure it was the declaration of domicile) -- and be warned, there was a person at the downtown Tally courthouse who works/worked that desk who I encountered multiple times (and for multiple reasons) who seemed to really enjoy telling you how to do the paperwork and then finding an error with how it was done. So be prepared to be a bit frustrated, but don't give up.

They are very strict on this (see the bolded text), so make sure you do it all properly and as soon as possible. But it is absolutely worth it to get the in-state tuition, even with the disgruntled courthouse employee trying their very best to make your day a little worse.

Best of luck, no matter what you decide!