Here is what I ALWAYS do with customers and never had a problem. So you generate clish config in a file, say for example from current fw in expert (say if its master, though name can be anything) -> clish -c "show configuration" > /var/log/masterfwconfig.txt
Get it off fw from winscp (you can enable ot by changing admin shell to bin bash with command chsh -s /bin/bash admin) and once you have the file downloaded, copy bits and pieces until donw to clish of new fw, just ommit parts say for mgmt interface, unless you have constant console to it, and you dont care for web UI access till its cutover. Then, manually download recommended jumbo from cp site, install it, reboot, then ENSURE config matches from existing to new fw by getting config file with same command and comparing the differences (you can do this in notepad++ or even compare it free download tool).
If this matches, there is no way you would have any problems, trust me. I had done this too many times not to be confident 100% in the process.
Andy