Hi Jack
If you want to depreciate the new asset acquisition over 5 years, then the useful life has to be there in the asset.
In the base method of your dep key, you specify if "dep after planned life" should be calculated or not. Even if you make it yes, still it would require useful life to be there. But please try this once (base method 0007 or 0011 allows dep after planned life)
The best practice is to create a sub asset for this asset from As11. Add the desired useful life in sub asset and its done. You don't need to change anything in existing asset
Ajay M