How can we override amortization basis in order to change subsequent periodic depreciation amounts?

The customer has determined that a group of legacy assets (placed in service no later than 2004; current useful life: 600 months) underwent a life duration change at some unknown dates in their life in the external legacy system (no single date known). Their useful life used to be shorter than 600 months (reportedly 360 months) and these assets were depreciated "too fast".  A few are already  fully reserved as of 2014/12/31 and shouldn't be.

Since 2013/01/01 they have been depreciated in Oracle Assets (11i in 2013-2014; recently imported into R12) using a STL method and a useful life of 600 months  (hence YTD = Cost / 50 years).

The customer wants to change the depreciation basis these assets as follows: