Be careful. As Eldartford points out it is not rocket science but a lot goes into making a decent speaker. A lot of planning, listening and tweaking goes into making two drivers integrate well. I messed around with a two and half way system for about two and a half years a few year back (that is not why they call it two and a half way). Granted, I'm not the sharpest knife in the drawer, but I found out that it is not unusual to spend that kind of time on speakers. Guys who make good stuff most often measure their experience in decades. There are just a lot of things that go into it. The T/S parameters mentioned above are especially helpful in the base integration. Less so with the higher frequencies.
Anyway, check the frequency range of the new driver against the old. Basically, in simplest terms, the two drivers have to share certain frequencies and over-lap by at least a couple octaves. Whatever you do(as Subguru notes), do not ask the new tweeter to go lower than its comfort range. The natural/acoustic roll-off of the driver combines with the crossover's electrical roll-off so, for example, if you want a 12db roll-off you can't just use a 2nd order crossover and expect results.
As noted above, while it helps to initially check the driver's characteristics it is less than the whole story because the driver/cabinet combo has it's own unique characteristics and those are what really matter. You likely will not find any numbers on your proposed combo.
In addition, it is very hard to get matched drivers by some manufacturers and individual driver units can vary quite a bit.
Finally, after you tweaked them , even if it is good change, you are likely to find that you have decreased the value of your speakers upon resale if resale is an issue.
Nonetheless, if you have some spare time and want to learn something, go for it. It is just very unlikely, IMHO, that you are going to do something simple, like drop in a new set of drivers and make a change in the xo, and improve the sound much. More likely to be a long process and you want to be in a state of mind to enjoy it.