When we upgraded from WAS 7.0.0.15 to WAS 8.5.5.11, we did an in-place install of WAS8. We had to bring down WAS7 so there wasn't any conflict with port bindings and port assignment. We assigned the same default ports. Then there is a tool from IBM (included) that will migrate your profile. You may need to look at JAVA SDK version requirements, memory requirement changes, and possibly resizing JVM for deployment manager, application, etc.
https://stackoverflow.com/questions/41249153/is-there-a-way-to-upgrade-websphere-8-5-5-10-to-version-9
You'll have to do a new install of v9, and then migrate your configuration from the old to the new environment. There are a few different kinds of config migration, but the main overview is here: https://www.ibm.com/support/knowledgecenter/SSEQTP_9.0.0/com.ibm.websphere.migration.base.doc/ae/welcome_migrating.html