I'm sure this question is going to be asked a million times over the next few years so I'll put this here as a reference.
My 'scope says "DHO804" on the front. I've modified the bandwidth and memory options.
Here's my new "About" screen:

Here's my new "Options" screen:

Here's my new memory depth with 4 channels enabled:

Here's my measured bandwidth:

nb. I'm using the bandwidth math from
here and Martin72 confirmed the "100Mhz" bandwidth with a 200Mhz sine wave
here.
(I don't have a SIGGEN that goes that high so I can only do rise times

)
All changes are harmless and 100% reversible. There are NO side effects.
Notice the difference in rise time and the shape of the yellow waveform between the last two images. That's because 321.5MSa/s isn't enough for a 197Mhz 'scope, ie. I'm seeing some aliasing.
(Solution: (a) Be aware, and (b) Turn off some channels when viewing fast signals and see if anything changes. If it does? Aliasing!)