I'm thinking of getting this mod to make my a town a little better, but I'm afraid once I use it in a save it will make changes that I wish not to have. If it does make any immediate changes right away, what are they?

My experience is that the mod will almost immediately start moving inactive resident sims, those not currently controlled by the player, into careers that might be more in line with their LTWs and will start coupling up single sims it "thinks" should not be single fairly quickly. This is all controllable by way of the mod's options though, and unwanted things like couples' status and careers can be undone by way of MasterController. Also note the mod is totally removable if you don't like it.

The mod won't suddenly move dozens of households into or out of your world or anything like that, resident immigration and emigration are both off by default and immigration doesn't work that fast even when it is on (it's at most one sim household per cycle) unless the player invokes the Rapid commands on purpose to fill an empty world.

What you might want to do is introduce the mod on default settings, turn its progression on, and see what happens on a copy of your ongoing game before committing to it, even short-term.

Sounds good, thank you!