Is my memory serving me badly or am I right in thinking that it used to be possible to continue updating ACR even after new versions of photoshop came out.
You can upgrade ACR within the same major revision number ; ie with PS CS4, which shipped with ACR5.0, you can upgrade to ACR5.7.
You can't upgrade PS CS4 with ACR 6.2, it needs CS5.
or is it still possible to download an updated DNG converter to use new cameras with older versions of ACR?
Yes it is!
The DNG converter is independant from PS and free ; the DNG produced can be read with ACR2.4 or higher IIRC.