In most cases you do not have to start over. The only thing you might miss is some of the new content.

There is no good way to turn off updates, but....
You can copy the game and scenario to a local directory and play it off line. Several people do this. Then the game will update, but not your "local" copy. If you search the discussions, you can find the actual procedure, Or someone might chime in here and tell you how.

One way to do this is to copy RE2 from the workshop folder. Find it under Steam's folder tree
it will be something like this:

[drive, likely C]:\Steam\SteamApps\workshop\content\...\3143225812

where the "[drive, likely C]" is the drive you installed steam on. The workshop ID for Reforged Eden 2 Beta is the 3143225812 number. Copy that folder and it should include everything under it, to:

[drive, likely C]:\Steam\SteamApps\common\Empyrion - Galactic Survival\Content\Scenarios\

be sure to rename it from 3143225812 to something like RE2MyWay or whatever you want to call it. If you don't rename it, it wont work as it will simply try to use the original location under the
workshop folder.

In my example, when it's copied and renamed the folder path would end up looking like:

[drive, likely C]:\Steam\SteamApps\common\Empyrion - Galactic Survival\Content\Scenarios\RE2MyWay\

Then run the game, under "start new scenario" you should now see a scenario option as RE2MyWay (or whatever you named your new folder).

Hope this helps. If it's not clear let me know and I'll try to describe it in more detail.

I have played the same save for some time and it was working fine. Just recently the update had some new stuff that was not showing up, and it looked interesting. So I have restarted, now. (But backed up my old save.)

I hate losing all of my stuff, too. But you can save your bases and ships by making blueprints (local or shared) and building them new.

You can also copy your ships/bases to your new save game. Spanj has a video on how to do that, though it's a bit old now but I've used his technique a month or so ago and was able to copy a base I liked to a new save. While a blueprint works just fine, a copy of the base should bring all the things you had in it over too. At least it did for me. Look for "TRANSFERING EVERYTHING TO A NEW GAME!" in youtube.