You aren't wrong, it is just that there are 3 "official" LUN types.  Traditional LUNs as you state are built on a RAID group.  These are thick provisioned, but aren't "Thick LUNs".  Thick LUNs are pool LUNs which are not thin - don't have the thin property box checked at creation time.  Thin LUNs are pool LUNs which are thin - do have the thin property box checked at creation time.  I think you are just mixing up the property of being thick and thin with "Thick LUNs" and "Thin LUNs".  Traditional LUNs are thick, but they are not "Thick LUNs" because "Thick LUNs" are pool LUNs.  Confusing enough for you? [IMAGE]  I usually distinguish with "Thick Pool LUN" if necessary but to each their own.  Your other question, can you change from thick to thin?  Yep, but not just by unchecking or checking the box after creation (you actually can't check/uncheck the box afterwards). Thick LUNs and Thin LUNs are different with how they are provisioned on the back end.  In order to convert, you would need to create a target LUN of the same size and desired type, and then do a LUN migration to it. 
Hey there  right know I am studying for the ITSMv2 Excam and I am reading the book about it. On page 82 there is a chapter about traditional LUNs and thin LUNs.  Traditional thick LUNs are mapped to a traditional RAID Group and expanding those would be a MetaLUN. So far I've no issue with that BUT beased on the book a Thin LUN resides in a Shared Pool. But from my experience with working on EMC VNX i can create a traditional tick LUN in the pool as well and expand it with capacity out of the pool. If i want to use Thin i can say this is a Thin LUN by klicking "Thin" and allocated 10 TB even if i am using only 2 TB. Am I right? Or am I wrong? Are all LUNs in a pool Thin LUNs even when "Thin" is not ticked? Can i change a LUN in a pool to thin and thick all the time? Are there two meanings of thin in EMC environments?  Thank you for clarification and greetings,
Preparing for excam: Questions about ISMV2