Well it depends on your old SSD, if its a quality one like Samsung then you won’t notice any difference to an M.2.
When i switched to M.2 on my latest build the loading times were maybe 1sec faster if at all.
In game it won’t have any effect if you are already on a SSD, not like switching from a mechanical HDD were the characters pop in one after another in Valdrakken for example 
Tldr; no difference for WoW 
You will notice a difference for some newer titles with direct storage support though and also the biggest benefit is always windows being more responsive.