There is clearly a point in Alzheimer's, and other neurodegenerative diseases, beyond which the damage caused by the condition is irreversible. Neurons die, and in large enough numbers to destroy vast swathes of information held in the brain - the very foundation of who you are, and the vital components of systems needed to live a normal life. All is not gloom, however. Studies in past years have suggested that up to that point, much of the loss of function that accompanies Alzheimers is in principle reversible:

Some evidence suggests that the worst effects of Alzheimer's disease can be repaired - that memories are not destroyed, but rather become inaccessible.

Another recent study adds to this picture:

Amyloid-beta and tau protein deposits in the brain are characteristic features of Alzheimer disease. The effect on the hippocampus, the area of the brain that plays a central role in learning and memory, is particularly severe. However, it appears that the toxic effect of tau protein is largely eliminated when the corresponding tau gene is switched off.

Researchers from the Max Planck Research Unit for Structural Molecular Biology at DESY in Hamburg have succeeded in demonstrating that once the gene is deactivated, mice with a human tau gene, which previously presented symptoms of dementia, regain their ability to learn and remember, and that the synapses of the mice also reappear in part. The scientists are now testing active substances to prevent the formation of tau deposits in mice. This may help to reverse memory loss in the early stages of Alzheimer disease - in part, at least.

For yet another consideration of early to mid-stage Alzheimer's as a form of dynamic blockage of memory access, you might also look at the effects of some newer anti-inflammatory treatments:

The [study from 2008] documents a dramatic and unprecedented therapeutic effect in an Alzheimer's patient: improvement within minutes following delivery of perispinal etanercept, which is etanercept given by injection in the spine.

Putting aside a discussion of the mechanisms by which this happens, the very fact that it can happen demonstrates the possibility of reversing the worst aspects of Alzheimer's. Thus memories and the working structures of the brain must remain largely intact until fairly late in the progression of the disease.