Is there a way to work on the budget files 2 months prior to the year end?

Yes, but in Excel!

Currently the answer is no. That answer will change in Ledger V9.0 expected to ship towards the end of this year.

You can always "export" the buget figures to Excel, do your adjustments there and once you close your year "import" your modified budget.

For a more detailed explanatio of the process Andrew and Michael described, check out Adagio Budgets in Excel

Once the old budget is exported in to excel,
are the "nifty" budget tools available somehow???

Wouldn't the process be better to open up a temporary new year; allowing visual access of current year values, and current year budgets,
then modify to a new budget....

Then name the temporary new year as budgetYY, then export into excel and import into your new year when the past year is finalized.

???

There are advantages to both ways:

Some people like the tools in Excel, like the ability to copy data across columns (when the amount is the same every month).

You can also create a second Excel sheet linked to the first one with a calcuation (previous amount * 1.05 for example) where needed.

However the tools to enter percent changes from current actual, or to distribute an annual amount across the months are not so easy to duplicate, so if you like these, your temporary database for budgeting should work well.