I suggest that the first thing you do is create a CSV database import file, using the CLX tag names (as much as possible) as the symbol name, then assign SLC addresses. Then program the SLC using the symbol names. Decide on a standard method of handling CLX tags (such as UDT elements and array elements, etc) that aren't valid SLC symbols, such as using an _ to replace a .
I've uploaded a spreadsheet on a previous post that should help. Fill it out and then import it into the the SLC program and you'll have address symbol names matching your CLX tag names.
See this post for the spreadsheet:
http://www.plctalk.net/qanda/showpost.php?p=258185&postcount=5
Once you fill it out, delete the first row and then save it as a CSV, the import the CSV into the SLC database.
One last thing: Inform the customer that there is quite a bit of work involved and that there will be an additional charge.