You can easily accomplish this with an import action. You can also do this with formulas and let them override but I'll tackle the import action here for you.
Step 1 - Enter the base values. In this case I've added a growth % by country by month based on your example.
Step 2 - Create a saved view that has the data you want to import. In this case we only want months, countries and the "base" assumption.
Note: I used system modules to filter dates and countries. You should do the same.
Step 3 - Create an import and use the saved view as your source. Under the scenarios mapping make sure you map base to whichever assumption you want. In this case I mapped it to Upside. If you want to also copy to downside, you have two choices. Run the import again but point it to downside or create another line item that is a copy of the base so you can map it to downside.
Step 4. Audit your results. We successfully added growth % to the upside as a "seed".
Jared Dolich - Retail, Wholesale, eCommerce Analyst
I believe, In your case, if you implement the way @Jared Dolich explained it should solve your problem.
However there is another way of achieving this by taking advantage of the functionality called "Bulk Copy". It is really helpful for one time activity. It might not work in your case because of below reasons :
This functionality can only be used by admins. So if the push from one list item (Base) to another list item (upside) is one time activity or end users don't need to do the push and the change is throughout the model (and not the module specific) then you as an admin can achieve this by a simple click
Step 1 - Go to Versions and Click on the Bulk Copy
Step 2 - In the dropdown pick the Scenario list and select your source and targets
In your Cases Scenario List -> Source is Base And Target is Upside