That wouldn’t be too difficult for an experienced modeler to do - what you need is to morph it. That way you will preserve the UV mapping and as far as possible use proportional scaling to preserve the texturing. How successfully it can be done will depend very much on the density of the mesh and the resolution of the texturing.
The texturing of the two dinos is very different, so would you really want to keep the original? Preserving the UV mapping will make it easier for you to re-texture.
OK - in case you don’t know, the process of morphing involves changing shapes without adding to or subtracting from the original mesh. Why I say it depends on the density of the mesh, is that the denser the mesh, the more difficult it is to do.
For instance, Melanorosaurus has considerably shorter legs, so you would need to compress the mesh in the legs on the Y axis and do so without getting edges overriding other edges. If the mesh is very dense, that will be a challenge.
What I would do, is to load the pic of Melanorosaurus the front grid - in the scene tree under the properties tab, there is a place to load the pic. Then load the Jobaria model and in front view centre it on the pic and do proportional scaling to get as close a fit as you can. For proportional scaling, use the universal manipulator and drag on the yellow box in the centre.
Now begins the fun part! I’d start with forequarters, that being the biggest difference - select all the polys of the forequarters and drag down to fit as close as possible without stretching the mesh - use soft select all the while and work gently.
I wouldn’t try to alter the curvature of the tail - that would be neigh impossible to do well. If it is rigged, you will have to adjust the rigging in your rendering app, then use the rigging to alter the curvature.
You select a bunch of polys by left-click dragging a rectangle, or use the losso, or select some polys and, holding down shift, hit the + button on the numpad to increase the selection and - to decrease.
You will need to do many iterations of altering the mesh and proportional scaling to get to where you want to be - be patient, work slowly and gently, save early and save often, each save under a different name so that you can go back to an earlier version if you mess up too badly! Remember, no deleting or adding to the mesh - unless you want to re-UV map it.
Once you have the new model completed, you can look at the size, because then it will all be in proportiona nd you can do proportional scaling to get the size right, if needs be.
One last thing - be sure that you have the right to alter the model - if it was released with unrestricted rights, you are ok, but if not, be sure to read the small print and if necessary get the author’s permission.
Good luck with the project - please post some progress pics