Seems the differences related to the truck's electrical systems are more related to the efficiency of it. The ER trucks have faster charging capability and also higher output to the motors, but those don't seem to be required to have the truck take a higher-capacity pack. I appreciate the point about how Teslas are built differently depending on pack size, but this is ultimately already a retrofit. Lightnings weren't made from the get-go for an electric drive configuration, so it doesn't at all seem infeasible that a retrofitted by design vehicle would be able to take further retrofitting. I'm absolutely confident the battery packs would fit, since the frame and body are the same. If the Lightning ERs got different suspensions from the SRs, it's still a truck designed to haul a heavy load, it's not a passenger car. Even if the extended pack weighed another thousand pounds, the SR truck's existing suspension is designed to handle that.
The most fundamental issue would be whether the truck would prevent the battery upgrade from being accepted by the truck's electrical systems. Even if that's the case, unless the batteries have some sort of security protocol where they're paired by serial number to the truck's electrical systems, you could conceivably have a hard switch to go from the original batter to the "reserve" battery, much as the old VW Beetles used to do with gas tanks before they got fuel gauges. I could see them programming the truck not to accept a battery that appeared to be higher capacity than it was designed for, but I doubt they would do that for a battery that appeared to be lower capacity, since every battery loses capacity with time and use.
There are other considerations, and I'm not an expert on this battery pack design, so I'm just spitballing, but it's a worthy conversation. Ultimately if the money makes sense, someone will build a business to do this sort of conversion, bottom line. There are going to be perhaps 100,000 SR Lightnings on the road by 2025. That's a huge amount of trucks that could use an ER battery. That's a lot of money to be made. Ford is already working on outfitting Lightnings with a higher energy density battery, so even the existing ER customers may find themselves longing for a different pack some day.