Rehabbing a Property That Already Has Tenants...? Help!

I am purchasing a 4-plex that already has 2/4 units occupied. However, I will be spending upwards of $75K to completely rehab the place and bring in a new clientele and rent level. Does anyone have experience doing this? Would the best thing to do be to buy the tenants out of their lease, or is there a way to do rehab work around them?

Any input is appreciated--thanks all!