I worked with SAP for a long time but in an area other than BI or B1 so my info is for your reference only...
(1) The key in UDT is typically GUID, a 16 digits or 32 digits string with type RAW generated by the system.
(2) The main table will have a field containing UDT key field. Notice the main table will use GUID(s) as a key(s) but this GUID may not be the same as the GUID in UDT.
(3) A and B fields from the old system migrated into the new one can be kept or deleted depending on what the values are for and where those are being used. Leaving both in the system won't hurt. If you delete them, the consequences could be huge for downstream data tables, programs using those fields. You defintely don't want to opon a can of worms.
Good luck with your project!!
