Corrosion on Dielectric Union of Water Heaters

We are selling our home and we received this report from the home inspector.

The home is equipped with two, gas fired, 50 gallon, State brand water heaters original to the home. We noted all four dielectric unions were corroded and show indication of previous leakages. We recommend the union be replaced, this may entail replacement of the water heaters themselves do to the extent of the corrosion. We also noted previous water leakage at the water shut-off valve on the left water heater and recommend the valve be replaced.

Is there a way to replace these four dielectric unions without replacing the entire water heaters?

Hard to say without seeing it. If the unions are severly corroded, it may not be possible to remove it from the water heater without damage, although a plumber can probably do it.
Valve replacement is straightforwars. How old are they WH?

Sometimes you can replace them.
I've done it before, but no guarantee when things get old. If it works, fine. If not, then oh well, it would have been needed anyway.

The dielectric unions are NOT part of the water heater, so they should just unscrew so new ones can be installed. My personal feeling is that they are strictly cosmetic to make inspectors happy, and do nothing to prevent electroysis. I would reconnect the water lines differently.