You can if the new theme has more and deeper theming. Good way to do a quick check is to check the file size of each. More MB would kind of tell me that theme has more and deeper theming. Nitro's themes have a ton of theming in them, so if Nitro's has more theming, some remnants of Nitro's theme will still linger around. This is all covered in the first few posts of this thread, check them out.
EDIT- My bad, I thought Jaekar updated his OP with this info. You should always wipe cache and dalvik cache when going from one theme to another. If you see remnants of the previous theme you will need to boot back to cwm, wipe cache & dalvik, flash base ROM, reboot, boot back to recovery, wipe cache & dalvik, flash new theme, reboot.