Don't even think of using the worn rear on the front. More than 3/4 of your braking comes from the front, and it's almost impossible to recover from a front wheel skid. The cost of a new tyre is peanuts compared with the damage a bald tyre can do. Bald tyres are also much more susceptible to punctures.

Don't worry about the tread pattern - it's not really an issue on hybrids. I've used a knobbly rear with a straight front for mixed riding - grip from the rear on gravel, and directional stability/good cornering from the front on the road.

Phil