Hello - I am just getting back into this...it was easier as a kid!

I have a Bachmann engine that is chugging along - I bought a new set of EZ Track and set it up properly.  If the engine is chugging...does it mean I need a new engine? 

The old engine does not have the newer coupler on it - my old cars do not either...they are of Tyco make.  If I buy a new engine, can you still get the older coupler?  If so what numbers/make should I look for?  Can you convert older cars?

What is DCC all about?  I have an Autopulse Transamp 1 transformer that seems to be ok - is it compatible?

Thanks for any help!