How can I for example empty all contents of layer 2, and then transfer all contents of layer 1 into layer 2?

In order to accomplish this, you would need to do a GDSII stream out without outputting a particular layer (such as layer 2 in this case), then do a GDSII stream in to a NEW EMPTY Library, remapping layer 1 to layer 2.
Here are the steps to accomplish this process:

1. Select Library: Criteria and create a new criterion, to be called all_except_layer_2. Select all layers and data types (blue), then deselect layer 2 only (white).

2. With all structures closed, select Link: GDSII stream out. Select the structure and click on OK. Then the GDSII stream out dialog box appears. Place a checkmark on the "Use criterion" field, and select the all_except_layer_2 criterion. Click on OK, Enter a GDSII destination file and click on Save. Then go ahead and close the library.

3. Once that is done, select File: New and enter the following line: 2 0 1 0. This means that you will be streaming in GDSII layer 1 data type 0 and placing it on dw-2000 Layer 2.
Then save the file as layer. map. (If you require additional information on remapping please go to the following page:1-23 in the "Data Conversion.pdf document" which is located under the Help Menu, by selecting "Data Conversion")

4. Then go ahead and select Link: Stream In. Checkmark the "Use Map Layer Table" field and click on OK.
Select the GDSII file that you created in step 2, and click on open. When you are prompted, give the name of the new library that you want to create. Then you will be prompted for your map file that you created in step 3.

When the above process has been completed, and you open the structure in the new library, you will note that the contents that were formerly on layer 1 have been transferred to Layer 2.