Upgrade problem in PosgreSQL below v.9.2

Error during upgrade with old version of PostgreSQL. Below the trace:

[SQL] ALTER TABLE IF EXISTS "core"."audit_log_bck" RENAME TO "audit_log_bck.old"

[Err] ERROR:  syntax error at or near "EXISTS"
LINE 1: ALTER TABLE IF EXISTS "core"."audit_log_bck" RENAME TO "audi...