There seem to be different opinions on this. It seem to be one of those things applied inconsistently (like so many things in Thailand). Some people claim you cannot get another visa while still holding a currently valid visa. However, none of those people have ever been able to point to any law or official statement that supports that. So, it comes down to hearsay. It seems to me, some embassies will allow it, whole others won't. Here is a report of a guy that switched from active METV to DTV, just as a positive example:
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