Wildflower plug plants can be popped straight into an existing lawn. It helps if the lawn is quite poor in the first place, with plenty of 'weeds' such as speedwell, clover, self-heal, plantain and bird's-foot trefoil. These will become part of your new meadow.
A lawn full of 'weeds', indicates that the soil beneath isn't too rich, which will suit the wildflowers and grasses well.
You can plant wildflower plugs at almost any time of year, as long as the plugs are available and the ground is neither waterlogged, bone dry or frozen solid. If you want to turn a small front lawn into a meadow, or want to add wildflowers to an area of bulbs naturalised in grass, plugs are perfect. They're usually sold in multipacks.