Should a water heater's anode be replaced?
The anode rod corrodes over time, and once it does, cold water is not properly delivered to the bottom of the heater which results in luke-warm water.  The simple answer is that it should be replaced as-needed.

On some water heaters, this rod is part of the cold water supply, so replacing it requires disconnecting the tank which can be a fair bit of work. Some heaters have what is known as a sacrificial anode rod that is independent to the intakes and exists solely to protect the rest of the water heater from corrosion - usually this one is easier to remove and replace.

If you have plans to shortly replace the whole tank, I would not replace the rod unless you suspect it is already corroded and causing problems.