Yes, you can upgrade within each family (804 to 814, 914 to 924) by buying upgrade keys from Rigol, or generating such keys yourself via a key generator script which is also available in this thread. Going from the 800 to the 900 series requires the other hacking approach, by modifying or replacing the vendor.bin file.
It should be equivalent whether you go straight to the 924 via its vendor.bin, or go to the 914 and then apply the bandwidth upgrade key later.
In either case, it is questionable whether the 924 model makes much sense. The actual, measured bandwidth for the 914 is already higher than specified by Rigol, namely around 200 MHz. And given the limited sampling rate of the small DHOs, they can't really keep up with that bandwidth once you enable more than two channels (and hence run at only ~ 300 MHz sampling rate per channel). So you will get aliasing if higher frequency components than half the sampling rate are present in the signal.
Frankly, I would stick with an upgrade to the 814 model. That one also has the higher measured bandwidth, and it won't annoy you with a display for the non-existent digital channels on the screen.