Can Firebird 1.5 UDFs be used with 2.0 server?
Short answer: Most of them
There might be problems with UDFs that try to access the security database directly, as that is not allowed anymore in Firebird 2.
Firebird 2 supports a way to signal NULL, and has made changes in the way UDFs are stored in system tables. Make sure you upgrade your databases using the scripts in 'upgrade' subdirectory of your Firebird 2 installation.
Also, if you use Linux, you might need to recompile some UDF libraries from source (this is a known issue with rFunc library for example).