How to grep decimal and periods in very sorted situations?

I have tables where the proofreading was not made and many numbers mix periods and commas and only the two last decimal numbers should have a preceding comma.

4.865,226.06  /  must be= 4.865.226,06

5,444.01                              5.444,01

21,005.345,29                    21.005.345,29

It seems easy to grep the last two characters (as the decimal are always finishing the line)

\.\d{2}+$

and later work the rest. But involves many steps... Poor Grepping.

How to improve this?