I don't know which version of Oracle you are running. Here is the piece of documentation you should read (this one is for 10g R1).
I'm not sure about WE8DEC, but UTF-8 is a superset of WE8MSWIN1252 (Windows Latin-1), so potentially you will lose data when passing from UTF-8 to WE8MSWIN1252. This is explained in the Globalization Support Guide (see the above link). However, if you had only Latin1-compatible data (only characters that exist in Latin-1), you should not lose any data in the process.
HTH & Regards,
ORA-000TK : No bind variable detected... Shared Pool Alert code 5 - Nuclear query ready .