Changing Level Height of Entire Model?

I think the best way to do is moving the whole model in one action to do this follow the next points:

1- Open an "elevation view" (north, east or any of that, even a section)
2- Go to "Manage"
3- at the "Project Location" on manage click on "Position" tab and choose "Relocate Project"
4- Now click on any place and move (like when you move an object) to the desired position i.e 100' in the Z direction of the project.
5- You will not notice any change, to get this you need to edith your "level family" and change the elevation base to "shared" this will change you current 0' 0" to 100' 0"
6- Do this also for your "spot elevations" select one spot elevation and click edit type, then go to "elevation origin" and change from project to "shared" this will update all your spot elevations since they're a family.

Don't forget to create a backup, just in case...

Hope this helps.
Fredy

https://www.cadlinecommunity.co.uk/hc/en-us/articles/201884812-Setting-or-Changing-Ground-Level-in-Revit-2015

1 - Go to an Elevation View2 - In Visibility/Graphics Dialog, under Site Category, turn on Project Base Point; Click OK3 - Select the Project Base Point object4 - Change clip state of point by clicking the paperclip object so it has a diagonal line through it (this will prevent the actual model from moving and will allow the Project Base Point object to be moved independently away from the model)5 - Move the Project Base Point (using Move tool) down 100'-0"6 - Turn the visibility of the Project Base Point back offThis will work when your Level objects are set to Elevation Base = ProjectIf your Level objects are set to Elevation Base - Shared, then repeat the above steps with the Survey Point instead of the Project Base Point.Recommend doing this in your template to avoid redundancy as you move forward.

or you can just specify coordinates at a point:
manage/coordinates/specify coordinates at a point
select the ground floor level and enter the known height of elevation in the dialogue box in the Elevation field (usually Finished Floor Level or Structural Slab Level or maybe a site datum) all the levels will change in accordance with the new specified level. same tool can be used with grid coordinates to accord with an OS map.
(tip draw a small model line from the grid intersections, this tool won't snap to an annotation/detail line)

maybe you can do that by a small trick you have to change the heights of the higher levels at first till you reach level 1 (in a descending order )

User can adjust the story level by bring up the Organizer palete and right click on Stories and select "Story Settings...", and they can adjust the height in the Elevation or Height to Next.

The story level will change and all the models placed on that level will be adjusted follow the height of the level.