Even if it was in violation of the lease for the tenant to change the locks, if I get the keys, no harm/no foul in my view.If I am not getting the keys, that probably means the keys are the least of my worries - legally or condition-wise.Exception: if I weren't already evicting (or the tenant wasn't already vacating), and if the lease wasn't near the end, and if the tenant was troublesome but paid on time, then I *might* use the change of locks as an excuse to terminate the lease (if the lease allowed me to do so) and then serve the tenant with a 30-day notice.  

Been at this for 18 yrs and have always taken the position:Can't trust the applicant to do this correctly and you may well get last years which is useless.I pay for the reports (cost of business) and get current data from Experian or Trans Union.I pull the CR, BG, Eviction reports ONLY after they pass muster:  Verified employment, income, prior landlord comments, so the expenses are well constrained.I also ignore the FICO score as that's the Banks view and I'm more interested in the 30,60,90 lates as that demonstrates their current habits.

Reacting to bad tenants, threats, or fear, will cause you more harm then if you sit back,  implement your lease agreement (according to the laws of your state), and let the courts do their job. 

As a matter of habit, I check the Mtg.s and liens on any property I'm considering.  

Full disclosure: I am a newbie with no entities of my own.Per the podcasts I've heard related to entities, there is no harm in flipping a few in your name, before worrying about asset protection.

My state has laws that make owner responsible for keeping the home habitable and making sure maintenance/repairs are handled in a timely manner.

If they are being considerate and giving adequate time for the old landlord to find a new tenant no harm no foul.

Make sure that you don't make it a habit.  

There is mold and mildew and different types of strains so more harmful than others.

IF they know of a violation, they might note it in their appraisal and depending on how persnickety the lender is, that could be a PITA but unlikely.If the municipality does a code check to issue a certificate of occupancy/habitability before closing, THAT could potentially throw you off.