Asset master mass changes using Substitution Rule?

Hi experts

I have to change depreciation keys for hundreds of already created assets (to enable lease interest postings where were not set up earlier). I have 2 questions regarding this:

1. Why do we use substitution rules for asset master data mass changes instead of LSMW? This may be a stupid question but I never used substitution rule for master data mass changes so there's something that I don't know so it would be nice to find.
2. So I have hundreds of assets all belonging to a single company, different classes and having different existing and target depreciation keys. Could anybody guide me how to develop substitution rule for these.

There's no change in depreciation rates or asset lives in new depreciation keys so there's no question of depreciation recalculation by the system. We just want to do this for lease interest posting.

Thanks in advance