You can move the landscape tiles up and down and not have a gap, but you need to do so programmatically.
Once your tile has reached 65535 value you start the next tile at 65535/2 height.
Then 0 will have the value of the height where the previous tile ended.
Obviously, because the highest value possible in 16bit is Odd, you may just want to limit the landscape to 65534 and do the same with even numbers. Which is where the “programmatically” comes in.
This is useful to have higher “per meter” detail and avoid stepping on really tall landscape (think K2 from below sea level to top).
With that in mind.
Your solution may be as simple as exporting all heightmaps and starting a new terrain with a different Z scale.
Ofc, if the heighmap doesn’t get adjusted you will distort the terrain.
Chanches are you can just move your current terrain downwards and extend the vertical unused portion avaliable to the terrain by manipulating the data directly in GIS.
If you let a gis program keep the height values as they need to be and rebase the lowest point you cna pull this off quite easily.
If your terrain was not a properly formatted png16 file, you will have to do extra work.
You start by taking the current scale, and the documentarion provided ratio, and calculating your max heigh/lowest point to then rebase your normalized heightmaps in GIS without causing distortion.
From then, you just rebase the lowest point…
Absolutely do not ever do as @UnrealVerseGuru suggests.
Most programs do not natively support 16bit and will distort the values of the heightmap - often causing stair stepping since you go from 65535 possible values to 255.
Heightmaps are data sets, not graphics. You have to work with them in programs that can properly manipulate their data in order to prevent unrestorable corruption.
(Once you go down to only 255 levels and save you can toss the file. There is no recovering the lost detail).