How to partly update Cesium mesh terrain dataset?

I started working with mesh terrain to feed CesiumJS and I have a doubt. I have generated the entire planet using the SRTM1 DEM dataset from zoom 0 to 14. Now, I would like to generate a more detailed view for zooms 13-14 utilizing a dataset for some countries with more precision, let’s say, 5 meters resolution. This new dataset would be only for zoom 13 and 14. I can easily merge both datasets using GDAL virtual files to feed the Celsium Terrain Builder tool in order to generate the meshes.

My doubt is, to add this new dataset to the entire globe (in mesh format), do I need to generate again the zooms 13 and 14 for all datasets? Is there any trick to generating only the zoom for the new dataset and then merging it with the current dataset of the entire planet?

Any kind of help is appreciated.
Thank you!