Ok so explain the problem a little bit more. Let me see if I understand correctly.
You downloaded a model and started working with it on sculpting, you added a multires modifier on level 2. After you had sculpted it, you applied the modifier, which resulted in a very very dense mesh. Now on top of that very dense mesh you again added a multires and this time it would only reach 4 levels, because may the base mesh was too dense. Now the real problem was that when you tried to bake textures, since the base mesh is too dense it would crash on you so now you can’t move forward.
So you decided to transfer the sculpted detail to the original base mesh you had downloaded, because that one didn’t have a level 2 multires applied and maybe the baking would go on fine in that one. So since you didn’t want to loose all the sculpting work you did, you transferred the sculpting with the reshape function from the really high dense mesh to the original one and it worked fine. So now you have all the work you did in the original mesh that you could now bake textures in it. Right? That could be the end of it, did you try baking?
But then you thought hmmm, what if I can make it even better with less topology, let me add a decimate modifier an try to retopo on it. But with the decimate the face count doesn’t match so now you can’t transfer with the reshape. So that is a dead end, that wont work.
If you want to retopo, you don’t have to decimate, you can do your retopology on top of whichever mesh, because you’re going to get a clean new one either way.
