That's the historical reality. There's a good deal of variation, though, in the age of different groups.
Body plans are extremely stable.
Larvae which are free-living face assorted selective pressures and certain changes are quite common. For example, many gastropod lineages have gone from planktotrophic larvae to lecithotropy or direct development throughout geologic time. In bivalves, evolution of small adult size is strongly correlated with a change from planktotrophic larvae to brooding. Walter Garstang's poems on larval forms give several examples. Development proceeds in characteristic time sequences and directions.