This isn't anally-retentive true, but this week SpaceX tested Starship 24 with the thrust simulators for its Raptor engines, then it swapped places with a section of booster to pressure test the new design for the tank domes.
Ship 24 was installed on the modified mount on June 4th, just 12 hours after completing its first cryoproof. On June 6th and 7th, SpaceX then put the prototype through another pair of cryogenic proof tests, both of which appeared to be completed without issue on the first try. The first test even saw Ship 24 use its nose vents, suggesting that SpaceX may have filled and pressurized both its main tanks and a smaller pair of landing propellant or ‘header’ tanks.
At some point during either or both of the Pad A cryoproofs, it’s believed that the mount’s hydraulic rams were used to test Ship 24’s upgraded aft end by simulating the thrust of six Raptor engines. Like Ship 20, Ship 24 will eventually be outfitted with three smaller sea level-optimized Raptors and three larger vacuum-optimized Raptors. However, Ship 24 will be the first Starship to use new Raptor 2 engines, which are capable of generating almost 25% more thrust. At full throttle, Ship 24 could theoretically produce almost 1400 tons (~3.1M lbf) of thrust at sea level, just shy of twice the thrust of an entire Falcon 9 booster. Starship will be the most powerful orbital spacecraft in history.
In this photo from Mary, BocaChicaGal for NASASpaceflight.com, we see S24 being lifted off the test pad after the final test. Those vertical elements sticking up, two on the left and one near the right hand of the guy on the right side of the stand, are the thrust simulators. In reality, they're equally spaced at 120 degree intervals on a circle that represents the middle of the three sea level Raptors.
S24 was rolled back to the Shipyard to have its Raptors installed.
The domes have been around for a while, we talked about them back in March as part of an article on similar changes to the Starship nose cones, but apparently haven't been adequately tested. Or haven't been tested at all. Either way, a new test tank was rolled out to the pad area to test the new design.
In this picture, also by BocaChicaGal for NASASpaceflight.com, we see the new dome on the right vs. the three level design previously used. The new design is easier to assemble and leaves a few feet of room in the booster. That either can be taken advantage of or removed to save a tiny bit of liftoff weight.
Following the tank’s June 8th arrival at SpaceX’s Starbase launch and test facilities, it likely won’t be long before the tank kicks off its first (and probably only) test campaign. Like past test tanks, SpaceX may start with a pneumatic or water test to check for leaks and ensure basic structural integrity, or it will move directly into cryogenic pressure/stress testing with liquid nitrogen. While the main goal of any test tank is to learn about new hardware, the best-case outcome would either be the total survival of the tank (i.e. too sturdy to be destroyed with the available test equipment) or its destruction well past the maximum pressure it was designed to survive. As of 2020, Starship tanks were expected to operate around 6.5 bar (~95 psi) and designed to survive up to 8-8.5 bar (~115-125 psi).
So while they didn't exactly swap the ship for the tank on the same test stand, they swapped positions. This tank is simply a test item. When it's done, it will probably be sent to scrap out the stainless steel.
My guess is that S24 will have its Raptors installed and be sent back to the launch complex by the end of the week. The FAA website still shows the PEA to be complete on Monday, but every other time they slipped the schedule they didn't update the website until the day it was due to be released.