Depending on how you have IIS configured, you may or may not have to change the IP address associated with the Lawson website. In other words, if you have (All Unassigned) as the IP addressed, you don't have to change anything.
Hopefully you used a hostname when you installed Websphere and can just change the underlying IP address.
If not, you will need to change a bunch of conguration files in Websphere as well as visit the virtual hosts section, change the webserver ip address (if you specified it), and re-generate the plug-in.
Here are some links on what is involved:
You might find that it's actually easier to remove and re-install Websphere with the new hostname, and then re-configure Bouncy castle and re-deploy Lawson EARs.