Complex sentence: The Russian Air Force, which operates the largest number of Su-25s, had planned to upgrade older aircraft to the Su-25SM variant, but funding shortfalls have slowed the progress; by early 2007 only seven aircraft had been modified.
Split into simpler sentences: The Russian Air Force operates the largest number of Su-25s, and has plans to upgrade older aircraft to the Su-25SM variant. However, this process has been slowed due to a funding shortfall, by early 2007 only seven aircraft had been modified.