Swapping from Steam to non-Steam client?

[Recaldy]

I purchased the physical CE edition of 2.0 for PC, installed and play it to this day. I bought the game off steam and launch through that everyday and it plays just fine. I haven't used the CD key that came with my Steam version. I don't see why it won't work once I slap on Heavensward content to it from SE. (Digital CE I plan on buying.)

[Havenchild]

I purchased the physical CE edition of 2.0 for PC, installed and play it to this day. I bought the game off steam and launch through that everyday and it plays just fine. I haven't used the CD key that came with my Steam version. I don't see why it won't work once I slap on Heavensward content to it from SE. (Digital CE I plan on buying.)

It's likely a lil different since you essentially bought a separate steam version as well (which allows it to show up on your library as an official steam purchase).
In your case you would still be able to play Heavensward whether you purchased a version from SE or from steam because you have both versions of the game already. CE benefits are also saved onto your account so no matter what client you run on any software these benefits do not disappear because you are still using the same account.

Steam kinda just acts like a license holder for the client, while the CD Key is used on an outside service aka. mog station but since you already have an activated account, the client naturally verifies this and allows you access without needing to activate the new key.

However a player with a windows version of FFXIV: ARR would not be able to buy Heavensward off of steam and have it work, as they don't actually possess an official steam version of the game.