Yes, you need a new IEP; I'd share the new diagnosis with the special ed director in writing, and request the meeting. Usually the IEP is re-done early in the year at the new school, to reflect how they do things. You may want to give the special needs preschool a call and ask how they usually proceed.

An IEP must be updated annually; it can be updated more often depending on the need.

DeeDee