Hi @andrea1, welcome to the forum 
Our original enclosure tubes and flanges are different to our new Locking Enclosures, so an original flange will not interface correctly with a new tube. The original end cap is compatible with the new flanges though, so if you buy a Locking Flange to match your new locking tube then it should be possible to transfer the end cap to the new flange.
The depth rating is determined by the weakest link. Aluminium locking enclosures can have a higher depth rating than acrylic ones, but the 300 m depth rating for the full vehicle is also reliant on having WetLink Penetrators for the cables (which will not be the case if your vehicle was purchased before mid September 2021). If your vehicle currently has potted penetrators then achieving a 300 m rated depth will also require replacing the potted penetrators with appropriate WetLink ones.