I would not use this method for a plant that grows very deep roots and/or for a plant that will live in the same pot for several years. In that case I use all new soil and put the old stuff in the compost heap. This is particularly the case if the pot with old soil was growing true annuals. They tend to not have deep roots and the upper soil will be more used up in the sense of lost nutrients.
Otherwise, this is a good idea and I in fact do so myself in many cases.
