renegotiate rtp <--> srtp in mid call

Hi All,
Can we renegotiate a rtp call to srtp and vice-versa in midcall? May be
with re-INVITE or UPDATE message? Is it a valid scenario if srtp is not
mandated?

In my scenario, server and client both supports RTP and SRTP and initially
call is negotiated to srtp call. Midway, server is sending a re-INVITE
asking to change to RTP.  What should client do in this case? Accept, as
SRTP is not mandatory or drop the call?

Which RFC should I refer for this scenario and what does it say in this
case?

Regards,
Sunil