First of all, I'm wondering what you expect from the new ID. Is there anything important that you cannot do with the existing ID?
As you say, these IDs are "coming from client". So, if there were any discussions about, say, data issues, you'd always have to refer to their IDs and not to yours. Also, updates of the data would contain the original type of IDs again and again and your conversion would have to work on these unknown future IDs as well.
If you really want to do it, you need a conversion which is guaranteed to be 1:1. I think a translation table, i.e. a dataset containing old and new IDs as its two columns, would be the safest way to handle this. You could easily check it for duplicates, amend it with future IDs, use it for the conversion (in either direction) by match-merging, possibly create a SAS (in)format for convenient in-line conversions etc.
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