I'm wondering if anyone out there knows how to do this. I want to change from one preset to another in the middle of a song. I'm trying to do this with Valhalla Room. I can see all the parameters in automation, but nothing as simple as just changing the preset.
If anyone knows another way of doing this, mid song, that would be great.

I don't recall - can you turn an effect on/off with automation? If you can, why not insert two copies of Valhalla Room (great plugin by the way) each loaded with a different preset. At the point you need to change - automate one off, and the other on.
 
Another option. If Valhalla is  on a bus (fed with sends) create a duplicate with a different preset. Automate the "sends" from the tracks that use it. This would likely provide a smoother changeover - if smooth is what you are after.

Not sure it is possible to automate preset change. If the effect is at the end of the chain (reverb usually is) create two sends with the FX on each send and use automation to control the sends

ValhallaRoom does not have an on/off but it does have a mix control which could be used to switch from one instance to another or put the plug-in in an FX Chain in the ProChannel. FX Chains in the ProChannel have disable/enable automation.

Great ideas! Problem solved!
Thanks!

Or just put it on another bus and automate the sends.
 

Don't overlook the possibility of "morphing" from one patch to another if there aren't too many parameter changes between presets, and the parameters can be automated smoothly. Sometimes with guitar I just increase the drive to go from a rhythm patch to a lead patch, and change the EQ a bit, over a measure or two.