You’ve got two choices and they’re both messy. All commercial heaters have the anodes under the outer cover and several makes have them under the inner cover as well. Take a look at my commercial water heater section to get an idea of what that means.
Rheems are troublesome for maintenance. Most commercial heaters are top-plumbed with dielectric unions, but getting them disconnected is hard and getting the covers off is harder, as there is little give and the unions themselves are too large to pass through the holes in the cover that the piping goes through.
Side-plumbing the heaters is the best solution. If the current tanks are of the “universal” type, there will be ports on both the top and sides and they can be replumbed from top to side. If they are dedicated top-plumbed, though, the plumber will probably have to cut the plumbing and put in stainless steel flex lines. Or wait until these break and then put in side-plumbed.
With the plumbing on the side, the inner and outer covers can be easily removed to reach the anodes.
The plumber should also consider opening the cleanout hatch and removing as much sediment as possible.