The amplifiers are different between the L1 models, so the speakers aren't interchangeable. If you were to upgrade, a new power stand and new speaker array would be necessary. It's also worth pointing out that L1s are sold in packages which will, at minimum, have a power stand, array, and bass module.
Regarding the L1 Model 1S and Model II, the difference is mainly two things: the amplifier and the number of speakers in the array. The Model 1S has 12 speakers versus the 24 in the Model II (which is like your Model I).
The Model II is rated for audience sizes of up to 500, while the Model 1S is rated for audiences up to 300.