I have a Marklin HO scale starter set from 1967 when I was a kid. I would like to expand for my grandchildren.
I understand the track has changed. Can I buy new track and transitional pieces to use the old track also?
Your starter set will have the track with a metal roadbed. We now refer to it as M-track.
The Marklin H0-track types which are being used now are the K-track, and the C-track. Both track styles have transition track pieces, so that they can be used with each other, and also with the older style M-track.
Most people tend to use the C-track. It has a roadbed, similar to the M-track. It is very sturdy, and has double electrical connectors. This helps in getting reliable (digital) electric signals through the track.