It appears that the Flotsam foundation can only be replaced / upgraded with the block(s) that came with the Isle of Siptah. We (My friends and I that play on our server) have only noticed the upgrade available with the Stormglass Foundation that you learn with the Stormglass Architect feat (level 30).
I do not know, off the top of my head, of any other block that strictly comes with the Isle of Sipta. If there are any other blocks related to the expansion, it would stand to reason that those can also upgrade the Flotsam blocks.