Hi. Long time lurker, first time poster.
So, Hidden Springs was the world that my current legacy family started in. The second generation moved to other Traveler worlds and now only the founders live in Hidden Springs. I would like to have an empty version of the world with only a few inhabitants to represent an earlier timeline, but also be able to travel to the version where the founder lives.
If I;
1. Start a new game in HS and set it up as way back HS. Run Total Annihilation, bulldoze the lots then repopulate with Progression, assign a basecamp and buyable lot and exit.

2. In the save-folder for my current save, make a backup of the NHD file for HS and put it in the dreaded "someplace safe" and replace it with the HS NHD from the way back save. Switch it back when I want to travel to present day HS.

My question is, is this likely to bork my save with broken relationships etc? or is there a way to rename a world so you can have 2 versions active?
Or would it be better to move the founder out of present day HS and do the annhilation on the current save so that MC can take care of all the broken links.

Hope that makes some sense, Thanks.

Makes perfect sense and it's a very interesting question, but no I'm afraid what you are proposing will not work well. The problem is that both the current time version of HS and the one you are trying to establish in the past are both actually the same world. There is no way to change the underlying ID of a world that has already been played and continue to use it in the same game -- the IDing of a world is a CAW process, or at least something that must happen when a world is being designed prior to ever being played.

The problem for practical purposes is that from the vantage point of your other homeworlds, the data for HS is being held pending in the TravelDB file. If you swap instances of HS around, the game won't understand that so relationships to sims back in HS will suddenly point in all random directions up to and including your sims having what are now random toddlers, bonehildas, or wild horses for grandparents.

The point is that worlds carrying the same internal ID as each other should never be nhd file swapped in and out of the same ongoing game as all kinds of horrible things can happen. It is fine to add nhd files that represent worlds an ongoing game has never "seen" before, this is useful for a variety of reasons, and it's fine to remove an nhd completely if you have no intention of ever playing that world or its sims again within an ongoing game, but it's the swap in and out thing that causes problems.

Moving your founder out somewhere else and then changing the nature and population of your existing HS so that it resembles more of its own past would be one and probably the simplest way to proceed. Another idea would be to find a totally different world, a player-created custom world, that is not HS but was inspired by it and possibly resembles it a bit and use that for a second HS-like world to be added to your game in addition to the original one. I don't know if such a HS v2 or HS Lite exists out there, but perhaps those here more in tune with TS3 custom world designers' products might.

Thanks for the super quick reply igazor. You are always so helpful and I appreciate that.
Yeah, that's what I was afraid would happen. So if i were going to take off and nuke HS from orbit..
*The Founder could reach maximum safe distance by moving to say, Starlight Shores with the household and anything I wanted saved in family inventory (but NOT invisible friend dolls, nope).
*Homeworld switch the Founder (or is that necessary? Maybe just return from vacation to an empty world ala twilight zone)
*Then I Travel back to HS without a Sim and choose a rando to be the designated survivor and make them active.
*On the courthouse choose Master Controller\ Sim \ Advanced \ Total Annihilation.
*Regress the Infrastructure
*Have the designated survivor Travel without sim to Starlight Shores and
*Make the Founder active and either move back to Hidden Springs or return from vacation depending.
Does that sound right? This is only the second generation of what I am hoping will be a 10 generation legacy. This seems pretty unlikely considering my tendency to mess with things until I break them but a girl can dream.

I'm not sure why you need to remove your founder from HS in order to TA everyone else and do the regression thing. The command can be run by sim and regardless it never affects the actively being played sim (or at least it isn't supposed to because that might cause the game to "end"). But maybe your storyline just works out more cleanly that way?