In this situation, only the new frame will appear .
Say there are two themes you use ... dayDream and soft7. Put the both ExplorerFrame.dll files in system32. When using soft7, name its frame as explorerframe.dll and the other two are explorerframe _original.dll and explorerframe_DayDream.dll .
While changing to DayDream theme, first rename the existing one say explorerframe_Soft7.dll; and then rename the target one as Explorerframe.dll; and restart explorer .
This is manual process . There are a few tools that can do it , such as Browsing deviantART
, Windows Themes Installer v1.1 by =Kishan-Bagaria on deviantART
etc, but I do it manually .