Hi Pascal,
As your question is big the answer can be very big as well. This area in BW is also not small. However I will try to put some points that will be of interest to you.
Few things to note in this case are
1. When a new dimension is added to a cube the Fact table(or E table) gets changed. This is an easy process as the new field(dimid) will have 0 for all the entries (different story if you are adding Line item dimension). So the new fields added will have data only after the date of transport. This needs to be communicated to your users.
2. You BIA Indexes should be turned of for the cube before the transport. So that when the new fields (cube) are transported system doesn't update the index.
Now to answer your original question, To save your cube structure or any other bw objects from Production server for a restore you can follow the below steps.
Option A
1. RSA1-Transport connection
2. Instead of SAP Transports select XML -- Exports
3. Select your cube or other objects and click on the 'Export as XML' button
4. This will save your file locally on your harddrive
I have done this before but not on Production server. So do some tests in your dev before relying on this.
Option B
1. Change the structure in Development and transport again to Prod
As the volume of data is very large I would not go with either A or B. I would also not expect any issues in Production as well. Only thing is users need to know that the report has to run from day of Transport.
Regards,
Sujit.