what do you mean by time-dependent deprec key and useful life? Once you have set these parameters for an asset master record you should not keep changing around your useful life and deprec key every month/year etc. These parameters are set for lifetime of that particular type of asset. You can only change these parameters in cases like if u have capitalized it in a wrong asset class or there is a depreciation rate changes.
hope this clears your doubt.
No offense... but it's right there in the third image!!!!!!
To make the UL change you have to access the detail depreciation area screen for the asset. Notice the More Intervals button at the bottom of the image below. After pressing the More Intervals button, the following prompt appears requesting the start date of the new interval.