Migration to new hardware with advanced model number

I have been working with F5 for the past several days with no resolution.The old F5 is 3900 and the new one is 4000.One of the suggestions was to just copy/paste the bigip.conf file and then try tmsh load /sys config.The idea being bigip.conf does not have any device specific entries like IP address or hostname(which are in bigip_base.conf).Did not work.Then I tried mount -o remount,rw /usr followed by vi cs.dat to exclude bigip_base.conf from the archive.That did not work either with specific complaint about bigip_base.conf. At this point I am thinking the only way will be to take a full archive and then do a restore using tmsh load sys ucs filename.ucs no-lecense no-platform-check.However as this will result in duplicate hostname/IP etc this will have to be done by taking the old F5 out of the prod network. So doesnt look like there is a way to transfer over the iApps/virtual servers/certificates etc and then manually change the device specific entries. Any suggestions/actual experiences please share.