As with all my Heavy Support Artillery and Hydras, the gun and platform slide out so that they are all interchangeable. So yes, I have 9 Basilisk hulls that I can interchange out the conversions. So Hydras, Colossus, Basilisks, Medusas, and Deathstrike, (the types I currently have built) are all ready to change out for whatever I feel like throwing down.
Here are some pics of the Colossus.
Next up, to finish my Grotesques for Dark Eldar.