If you change the inside IP in your PDM you will lose the PDM connection !! I might be easyer to do this in command line?
Commands:
enable
conf t
ip address inside New-IP Netmask
write mem
example:
ip address inside 192.168.1.254 255.255.255.0
Check if there are any access-list that uses the old IP address.
write term
sincerely
Patrick