Assuming that the old tank had no substrate (i.e. the biofiltration capacity all resided in the filter) and the newly purchased plants don't start dying, you'll be fine. Otherwise, transfer the gravel as well, as this is where the main biofilter capacity might be. You can remove it later from the surface of your Eco-Complete once everything is seeded and stable. And be careful, adding plants relatively gradually, to make sure there's no mass die-off of the plant matter to overwhelm your nascent biofiltration in the presence of the fish. It is unlikely but could happen with, say, mail-order, emersed-grown plants.