James,

If your substrate is old - at least 4 months old - you don't need to
replace it to get better plant growth. If you want a different look
that's another thing.

What makes the substrate work are the bacteria that live in it. An
old substrate has that bacteria established. From what I hear Eco
Complete is truly a great product. It comes seeded with bacteria
which is what makes it work from day 1. Do the math.

If you are having problems with growing your plants you will have to
make sure that your water parameters are indeed what you think they
are. If you get the parameters right the plants will react within a
few days.

--Nikolay