He's elderly, has emphysema, smokes at least 2 packs of cigarettes a day.

Smoker's homes are the "best!" 

Its like selling cigarettes kinda.

This exacerbates the problem mentioned before regarding individual unit control, as well as raises an issue of where the return air is ducted from (i.e. if there is a smoker or otherwise unpleasant smell, that will get circulated through all units).1.

My tenants have complained a few times about how they have tried to start fights with them and on top of this, the neighbors smoke cigarettes/weed in their unit, which is causing the smoke smell to come through the walls into MY unit.

The new owners are "in-unit" pot smokers and it seems the smell is getting into the common areas of the building and into our unit specifically.  

But, based on the condition of the house and it's also caked with cigarette smoke inside the house I could see that. 

Hi Leah,If you move them backwards (ie. was due 15th Feb. now due the 1st Feb.) they will probably not have the money to pay.If you move them forwards (ie. was due 15th Feb. now due the 1st March) they may have the money to pay (unless they are irresponsible and spend it on beer, cigarettes and video games, etc... before they pay you).Good Luck!

He’s a house smoker and it still has some original shag carpet.

You get into problems with freedom when your freedom intersects with mine - like when all the smokers got kicked outside to smoke when the product of their addiction to nicotine - smoke - was proven to cause  health issues for those around them who don't have that particular addiction.