Rehabbing occupied units and moving tenants within the building

[Hiro Kitagawa]

Hi.  I'm looking into a small multiplex (3 unit) and that are currently 2/3 full.  The unit is quite old, built at 1902 and has a lot of potential for upgrades.  A couple questions - 

I'd think that rehabbing a currently occupied unit would pose certain risks, mainly damages to personal properties and injury to tenants.  With that said what is the general guidelines for rehabbing an occupied unit?  

Another option would be to rehab the empty unit first, then move the tenant into that one, then go to now-vacant unit to rehab.  Doing this twice would enable me to rehab the entire building.  All units are the same size.  However I'm not sure how to go about this as far as current lease term (whether lease would renew at point of the move) and potential incentives to the tenants to move.  What are some thoughts among you guys?