I believe the SR+ has the same number of modules, 4? The obvious way is to rewire and add interconnects so when charging they are electrically parallel as usual. When driving the interconnects switch the circuit layout so you have two sets of 2 modules in parallel, and the two sets are serial. Boom, V is doubled.
However this and all the other wiring and software tweaks and QA on the changes are far more work than that simplification, though. For example DU would need to be re-engineered from nearly the bottom up, electrically, electro-magnetically, and thus to an extent mechanically and definitely heavily on the controlling software, too. Look how long it took Tesla to tune the current DUs to their current state, years after shipping started.