@TaxGuyBill @Just-Lisa-Now- Bill just was reading about rev procedure 96-31 it is quite a bit of reading, but it seems you can now use this procedure to claim deprecation that you missed, and didn't claim all of the deprecation you should have. I guess you still file 3115, but it is automatically approved, but you have to state you are filing under rev prod 96-31. Here is a excerpt:

.01 Application of this revenue procedure. Except as provided in sections
3.02 and 3.03 of this revenue procedure, this revenue procedure applies to
any taxpayer changing to a permissible
method of accounting for depreciation
for any item of property that: (1) under
the taxpayer’s present method of accounting, the taxpayer has not taken
into account any depreciation allowance or has taken into account some
depreciation but less than the depreciation allowable (hereafter, referred to as
claimed less than the depreciation
allowable);