Is rebooting to a new software installed on a partition the same as running touch /service/mcpd/forceload and reboot?

Not sure if I got your question correctly. But I'll explain few points,

Every volume would have its own image file and its corresponding configuration files in it.

Whenever you switch your volumes and boot in it. The images and its text configuration files gets loaded in the memory too. Its like a load sys config action performed.

If you plan to install a new image on a separate volume and if your intention is to copy the running the configurations of previous volume to new one, there is an option to install a certain configuration from other boot location. Similarly the db value also could be changed if you plan to do via CLI.

Coming to touch /service/mcpd/forceload, this usually we perform when the bigip is having mcpd stuck or in a hung state. MCPD is the master daemon. Its like the box is dead as much.

Though you perform load sys config, if the mcpd is stuck. Its still not going to load the text configurations into your running memory.

Which is why we do a forceload. For which we have to touch the mcpd and its followed by a reboot.

So that this flushes everything and reloads the text configurations into the memory.