Yes, a DHO804 can become a DHO924 via a simple hack
I wouldn't do that.
There is no need, with the 900 series you get 2 more decoders, but he already has them anyway with his SDS2000Xplus.
Over 200Mhz bandwidth can cause problems if you don't remember that the sample rate on the Rigol is drastically reduced when using multiple channels.
If the seeker had no scope or only the DS1054Z, I would clearly recommend the SDS800X HD, as I think it is the best scope on the market for the price,
overall.
But he still has the SDS2000Xplus, so I would make a different decision and get a DHO800.
A DHO804 and "gently" upgrade it to the 814.
If it's all about the low-noise 12-bit, then the Rigol.
For a single reason, which I won't mention, you'll have to come up with it yourself.
If you know my review of the DHO800 and read my posts in the SDS800 thread, especially the last ones before I sold it, then you'll figure it out.
And I still think that if he wants to experience a general "over-all" improvement, then he should sell his two scopes and get an SDS2000X HD.
But that's not up for debate.