I don't think its possible to erase something from your medical record generally speaking. Alot of diagnosis are basically theoretical and thus unfalsifiable anyway (basically religion)
Technically its possible to get a change in diagnosis, ie a counter claim of misdiagnosis, but in practice this is really rare, because of clinical but-covering, psychiatrists not wanting to disagree with each other, and that roughly theoretical basis of most of the supposed conditions.
A really good example is hyperthyroidism. Its frequently misdiagnosed as bipolar (there can be near identical symptoms, high energy, mania, aggression, psychosis). But what if its proven, well after the your "episode", that you have elevated thyroid hormones? Do they then drop your diagnosis of bipolar? Well, no. They might, and generally would claim you have both, seeing as bipolar is unfalsifiable (ie more of a beleif than a fact), and they want to cover their own arses (you might have a relapse, and for some reason, they think thats more of a problem for them than you) . So even in a case of fairly clear misdiagnosis, they often do not remove the offending bullcrap.
Medical records in general can only be modified by the person that put the original thing in.
As to whether you can avoid your insurance company finding out? Well it depends on how centralised your medical records are in where-ever you live. If they are centralised to the degree that such information is centralised, then you have no hope of avoiding it, unless you disappear from the country.
If however, your medical records are not centralised to a degree where it can be accessed by anyone with permission, then you can simply get a paper version of your notes, throw away all mention of your diagnosis, and give what remains to a new doctor, or simply refuse to have your notes transfered, and tell them manually about any physical conditions, allergies etc.
Sorry thats probably all pretty disappointing. There's alot of people out there who just want to live normal lives, and have no need for mental health services.
This amusing guy however, if indeed his story is true, probably because it was early in his faked diagnosis (harkens back to the unfalsifiable thingo) managed to switch diagnosis :-
factitious-disorder/topic110733.html
"Some patients have a mental illness and then get well and then they get weller! I mean they get better than they ever were .... This is an extraordinary and little-realized truth" - Karl Menninger MD