I start Emplode 2 and, in Options, I change Riocar IP to DHCP.

First of all, you don't change the player's configuration in "Options". You change it in the "Configure Player" screen, in the "Networking" tab. We're talking about two different screens. The "Options" screen controls the Emplode discovery phase. It tells Emplode how to look for the player. It does not configure the player.

What I'm saying is... If you are connected to the player with emplode beta 13, and you are connected via Ethernet, the "Networking" tab is missing from the "Configure Player" screen and you don't have the ability to change its IP settings.

When I leave, I do all the steps in reverse, so Rio takes my home fixed IP again. Everything seems to work. (...) Now I'm running a b13.

If you're doing all of that from Emplode Beta 13, I don't see how. Whenever I'm connected to my player with Emplode Beta 13, I can't get the Networking tab to appear in the "Configure Player" screen.

And why is it that you say "everything seems to work" when the whole point of you starting this thread was to say that it didn't work and you wanted a fix?

Are you saying I wont be able to do that when I upgrade my Rio or Emplode?

I don't know. I'm not sure what you're doing now and why it seems to suddenly be magically working for you. Assuming you really are running Emplode Beta 13.

In any case, you will always be able to change the networking options with a USB or serial cable.

Finally, if you are hopping between two networks, one with a fixed IP and one with a dynamic IP, the work-around is to assing the player a fixed IP and then add an IP in that range as a second TCP connection on the dynamic-addressed computer. Details can be found in the FAQ here.