This is a common issue with migration to v2.1 and standard UDFs like ABS, ROUND, etc. The usual workaround is to quote the UDF name, i.e. DROP EXTERNAL FUNCTION "ABS", but it doesn't work for Dialect 1 databases.
Actually, the issue is not limited to migration only. Neither you can create an UDF that's expected to override the built-in function with the same name. And it's not about new built-in function names only, the issue is about any new non-reserved keyword introduced in the parser.
This limitation is caused by some UDF related tricks in our parser. But it can be easily improved to handle DDL properly.