you need to put a Z in the flight rules box (item 8) and then pretty much as you said -
CHANGE OF FLIGHT RULES (Maximum 3 characters)
The point at which the change of flight rules is planned, expressed exactly as in
2) or 3) above as appropriate, followed by a space and one of the following:
VFR if from IFR to VFR
IFR if from VFR to IFR
Examples: LN VFR
LN/N0284A050 IFR
in the route box (item 15)
Have fun!