It is very simple to change from the Coastal to the Central route once arriving at Vila do Conde
Arriving in Vila do Conde , you see ahead a former monastry on top of a hill. You cannot miss it. Also in front of you is a river with a bridge
Walk over the bridge and directly turn to the right. Then you are on the waymarked path of the Central route direction São Pedro de Rates. You follow the river for a while untill you cross the metroline to Porto center and Póvoa da Varzim . The route to São Pedro de Rates is fully waymarked
alternative to São Pedro de Rates by following the antique aquaduct are shown here above in this thread
if you want to keep on the Coastal here, go over the bridge, turn right and then inmediately right again so you go UNDERNEATH the bridge to the other side of the road and follow the yellow waymarkers into the center of Vila do Conde and Póvoa da Varzim and on to Esposende.