I walked part of the Camino in 2005 and plan on returning to finsih next month.
I still have my old credential from 2005. You guys think I can still use it or will I need
to get a new one ?
You can certainly still use it - many people take some years to complete their chosen route and get sellos on the same pilgrim passport each time they walk a section. To obtain the Compostela from the Pilgrims' Office in Santiago they will be looking specifically for sellos from the last 100 kilometeres travelled into Santiago.
If you run out of space with the credencial you already have either just get a new one or add on pages of your own making which I have done in the past.