Existing DVR memory replaced?

Looking into purchasing a DVR Expander. 

My question is will my existing DVR memory be replaced with the DVR Expander memory?

For example:  My existing Direct DVR holds 100 hours of HD programming.

If I buy a 1TB DVR Expander will I now have the ability to hold approx. 120 Hours of HD programming (DVR Expander)


Approx 220 Hours of HD programming (100 existing DVR memory + 120 DVR Expander)

Any help you can give me is appreciated!