I would not recommend changing the subwoofer in the LSi15. If you do, you need one that will be suitable for that cabinet size and port, and will be suitable for that specific crossover and be of equal output of the stock woofer. Otherwise, you will need to modify other aspects of the speaker to get it to sound right.
If you were talking about the LSi25, it would be easier to get away with changing the subwoofer because the subwoofer is powered, and consequently you would not need to worry about the volume matching (as that would be accomplished with the level control on the built-in amplifier). However, you would need the replacement subwoofer to be suitable for the built-in amplifier, and still need to worry about the cabinet volume and port being right for the replacement (or they would need to be modified in order for it to sound right).
Back to the LSi15: If you want more or deeper bass, just buy a good separate subwoofer (or two) to go with them. I like SVS subwoofers, but there are others worth considering.