I merged two already weighted objects. Is there a way to delete the old objects and keep the weighting on the merged object?

You can choose to delete the objects you merged in the Merge Dialog.
Or "Freeze M" and then delete the objects, should also work.

Thanks, that worked. I also have the problem where I cannot change the texture of the same merged object. Freeze M did not help for that. Is there a way to remove the texture (not UVs)? maybe that could help.

If the objects you merge have multiple materials assigned the merged object will have clusters with those materials applied, so you need to change the material of the clusters. You can select the clusters by expanding the object in the explorer and look for a Polygon Mesh node, the clusters should be parented to that.

Thanks, I just deleted all my clusters except my UV and weighting clusters, then I could change the texture again like normal.