Asset Register Query

Apologies if the answer to this query may be simple but I am fairly new to working with the Asset Register and so some parts are not yet clear.

If we have a group of assets that have been depreciated down to zero and are now held as Nil NBV on the Asset Register but we still own and fully use the assets what would be the correct accounting treatment for this going forward?

1- Should the assets have had say a longer useful life to start with and so depreciation would have taken longer? If this is the case can we just re-life them now?
2 - Should the asset now have a residual value say 20%?
3 - Do we just carry out a revaluation of the asset along with a re-life?

We are just interested to know how other authorities deal with the same issue and what the journal entries may be after the issue is dealt with? i.e do we increase asset value along with Reval Reserve as you would a revaluation or do we reverse depreciation value and thus increase the asset value?

Many thanks