It is near impossible to answer ths as I have no idea what role the old one has in the network.. nor what old means.. is it N or G?
Anyway.. as a general rule of thumb.
The name of the device is associated with its MAC address.. and since you are changing the MAC address with a new device.. all kinds of issues on the network are going to happen if you have the same name.
In theory it is therefore much better to start over afresh.. clean new setup.. new names for the new device. The little bit of extra work to connect your devices to it is worth it.
In practice as long as you install the device.. if you use the same name and password.. then shut off everything.. leave everything off for a few minutes.. then restart in correct order.. modem.. express.. clients.. it should work.. don't blame me if it doesn't.. see above for the right way.