What happens if i change from sillixx to something else and i now have too many devices installed on my ship and my deck space is over the max.

Some of your modules will provide no bonus.

And you won't be able to add any other mods until you get your deckspace back to a positive. I did that switch when I unlocked the Kronyn race, so I recommend you save your Shipbots and rank points (if you don't get many bots) until you have enough to cover the decks you will lose. DO NOT use them until you are ready to make the switch though, as extra decks will increase your damage cap.