Changing Asset Life using Amortize Adjustment flag issues

We are trying to make changes to the useful life of many assets (increased and decreased) and do not want the accumulated depreciation to be adjusted in the current period.  We were looking at using the Amortize Adjustment flag but on testing this we are seeing some rather strange results in that the YTD depreciation sometimes becomes negative.

All the assets use the STL method and we have this set up as Method Type: Calculated, Calculation Basis: Cost, both Depreciate in Year Retired and Straight Line Method are ticked.

Is there any documentation that explains what the impacts are when changing the useful life of the asset when using the Amortize Adjustment flag or can someone advise as to how using the Amortize Adjustment flag affects the depreciate value i.e. whilst the documentation appears to indicate there would be no impact on the current month's depreciation is the depreciation for the YTD adjusted to take into account the part year depreciation already processed.