There are mods that allow you to travel between different worlds if you have world adventures (since that expansion pack introduced the whole traveling)
Your families won't get deleted if you install an expansion pack, you don't even have to change the world if you don't want to. Expansion packs excluding Island Paradise offer content you can access in every world, provided you have the community lots for the activities that need them (for example, bars and night clubs in late night, and all expansion based community lots will be avaivable in the community lot bin in world editor) .If you got pets, you'd be able to get pets in sunset valley too. Or with supernatural, story progression will automatically convert some townies into supernatural sims ( you can turn this off too) and you can add an elixir shop yourself into the game. Even Island Paradise's content can be accessed in other worlds, it's just a tedious progress so I wouldn't recommend it. Some expansions don't even add a new world, like generations or seasons or world adventures (WA comes with vacation worlds), since they're more focused on filling other aspects of the game. I myself love building the different community lots into the worlds I play in to suit the world's architecture, my favourites being the medieval Dragon Valley and the nordic Aurora Skies (I'm from scandinavia so it feels the most homey to me haha).
But even if you wanted to move you'd still keep every item in your sims inventory & family ties, granted you take the remains of all deceased sims with you. However if you happened to have a sim with a best friend or extended family that relationship would be lost, unless you moved them in with your main family first. Whenever I have to move my Legacy to a new town I wait on all older generations to die and then move the whole extended family in with the heir and grab all urns, lol.
