Most of the beneficial bacteria lives in the filter and the substrate not the water, however transferring the water over to the 125 will help the fish acclimate. What I would do is transfer the filter media from the 75's filter (or even the whole filter) to the 125's filter, that might be enough to prevent you from having to cycle it depending on the size of the filters. As for the plants, it depends on the plants; hardy plants would be fine but more sensitive plants could have problems.