Rather than getting into the argument in the above linked thread, simply refer to the
FAQ (
http://www.blacklibrary.com/Downloads/Product/PDF/Warhammer-40k/7th-faq/Dark_Angels_v1.1_Oct14.pdf:
To become an Apothecary, he needs to be able to swap his Power Fist for a Nathecuium. He obviously cannot do that if he doesn't have a Power Fist (due to swapping it out already).
The
FAQ alters the selection process to allow a
TH/
SS guy to take a Cyclone as well (previous wording for the
TH/
SS was "replace all weapons", which gives you an order-of-operations problem similar to the linked thread, whereas the new wording works regardless of order).